How do you color under the lineart if you only got one layer? Not to mention, I make a lot of mistakes, layers make it easier to correct them, even later on
Okay, now I'm starting to think maybe this really is the reason...although a recolor tool doesn't sound like something that would be difficult to code; I don't see why most of the expensive art programs wouldn't have one.
And then, a lot of the other stuff I do is just masking...like, if I do the shading wrong and decide to redo it, I can just select all the shadow colors and revert them to base colors-- bam, we're back to flats.
Or I can select them and modify them with the hue slider...stuff you could probably do in any art program. =/
I get that it wouldn't be quite that simple with anti-aliasing involved...but I have done a little actual painting before, and even that I prefer to do all on one layer, because then if you don't like something you can just paint over it, or 'smudge it up' so it looks correct. Rather than having to flip through layers and redo an entire effect just to extend a shadow a little bit.
It makes it easier to layer, ha layer, things like shadows. Things won't get muddled together on one layer and you won't accidentally erase your shadows when doing your highlight. And then there's no picking a shadow color for every individual color in the drawing.
Edit:
Yea but with multiple layers, you can just delete the layer or clear the layer, and you get the same result. It's just a matter of process, you know.
As an add on, Multiple layers is really important for me because I like to color my lineart. It's just convenient to draw my lineart and come back later to color those lines on a separate, clipping layer.
i take joy and pride in making people uncomfortable with my obscene amounts of layers
also it's just easier to fix or edit stuff. i know there's tool or whatever in some programs but i like to keep things simple and organized. i'll absolutely merge things down if need be but i don't think i've ever just worked on a single layer unless i was doodling or something
felt that 
sometimes if I'm feeling really lazy, I'll have over 30 unlabeled layers (which is hell if I forget which layer is which)
Layers allow for a much more versatile, non destructive workflow. It's a lot easier to achieve complex, layered effects and lighting using multiple layers rather than a single one. I don't use as many layers as other artists I've seen, but a specific set of layers are essential to my pipeline. For instance, I'll have flats on one layer, then each light on a separate layer utilizing various blending modes, then shadow grading, scene light grading, then total scene tint to pull my palette into line. Inks go on top, then a folder of post effects like sparks, glow, fog, bloom, etc. above that, and final levels.
Layers are also just useful for splitting certain things out as I draw them. For instance, if I know certain inks are going to be color hold, like glass for instance, I'll split them out into a different layer as I'm drawing them. Then I can move that element around in my layer stack and recolor it as I see fit. Sometimes I'll lay down flats for various scene elements in different layers if I know I'll need to select an element later. For instance if I have a scene where there's a woman in the foreground against a foggy background, I'll put the woman in a different flats layer than the background flats so that I can just select the woman with a single click to mask out the fog in the post layers. Huge time saver if you're doing complicated stuff!
Multiple layers make easier to delete the stuff you need to delete if you make a mistake without deleting things that were right as collateral.....
The smallest amount I'll use is 2 layers. 1 for lines and 1 for colors.
But I'll work with multiple layers until I get a sketch I like, then merge together, then put the lines in a layer above, then clear the sketch layer and use it as a color layer.
For me it was a RAM issue. My machine just couldn't handle a file over too many layers / pixels big. Even if it was a 1500x1500 300dpi image, it would eventually have a RAM issue due to too many layers. Now I have a decent rig and I just use folders (folder for sketch, folder for color, ect).
But yeah, I could do stuff on 1 layer, but I likely wouldn't color it lolololol because I don't have a tool to color it without destroying the lines.
I think it depends on the person and your background. I was originally a traditional painter so I used to only paint in one layer basically all the time. I don't do that digitally anymore, mostly for speed reasons and certain techniques (I can actually get really layer heavy if I'm just trying stuff out and messing around) but like...you can easily paint on one layer. Especially if you are going for a painterly look. In fact I'll often just straight up flatten my image when I'm painting if I feel like I have too many layers going.
For stuff like comics where you need font and bubbles I use a billion layers, but that's another story and that's more to preserve them so I can edit better.
Like others, I personally use multiple layers to help differentiate things. So for a typical painting I'll have: skin layer, hair layer, clothes layer, background layer, particle effect layer, etc. I usually end up condensing them near the end though! 
I could make a landscape painting on one layer easily enough - it wouldn't look anywhere close to my best work, but if I planned out the steps I was going to take, it'd be possible.
But there's no way I could recreate my usual style with a single layer. I have so many glowy lights, and stars, and gradiating colours... I need my layer modes, and I need to be able to segregate and alpha-lock colours in order to shade the way I like to.
I do tend to work with way fewer layers than a lot of artists, especially given mine is a sci-fi comic which requires fancy layer modes for glow effects. That stems from using Procreate with large canvas sizes, since that app limits layers relative to canvas size, DPI and RAM. I have more wiggle room now, since I upgraded to a new iPad, but I still have to watch my layer use carefully.
Layers are just a tool to make your life easier. I use 70+ in a finished panel sometimes - especially if it has a painted background. 
I like being able to delete a layer if something isn’t working, or just nudge things around. I use multiply and Liner Dodge a lot too.
Don't have any comics out there yet, but felt compelled to show up on this thread nevertheless. I am primarily a traditional artist, which is the epitome of someone who only works in one layer. Even so, when I make art digitally, I end up with lots and lots of drawing layers. I believe a project I am currently working on has about 60 of them. It is not a problem if you name each one of them, though. It all comes down to me not liking to erase things but also not being a fan of the transform tool either. If I did a scene with three characters and one background, I would probably do:
Three layers for the background sketch (rough, kinda nice and I think I can ink over this now).
Four for each character (rough poses, anatomy, clothes and I think I can ink over this now), which amounts to twelve layers in total.
One for inking the background.
Four to ink each character (body, clothes, props, stuff I didn't get right ), which amounts to twelve layers in total.
Since I do a layer for each color I put on the backgrounds or the characters, the flat layers can go anywhere from thirteen to twenty. Since we are talking about four characters and one background, let's say I would settle down for seventeen.
Then there are five colonizing layers: one for each character and two for the background.
Then there's one layer for shading and another layer for applying light.
I also do a layer mask where I put a noise filter.
That would be about 53 layers. And that is for a drawing I already sketched on paper. I don't know if I have a problem, but that's what works for me.
Why are people so obsessed with the number of layers we use in digital art... Traditional art uses layers too, or do y'all think those watercolours gonna look pretty with just one splash of paint. 
Copics look horrid with just one layer omg! XD
But as for digital, I like being organized and be able to quickly make changes. And whether I use one or 50, I don't see why it should matter to people, you're just gonna enjoy my finished work so enjoy it. 
because it's easy with all the lineart and sketches tbh.
everything can have it's own layer, and we can go fix it without having to worry.
