Eh, my answer would be exactly what darthmongoose said, but mostly some of us really aren't in any pressure to improve. Some of us don't really push ourselves everyday to improve, we kinda just let time do its thing eventually, especially if you're self-taught. This is kind of how I've adapted to the concept of improving art quality, the way I try to improve my art is picking up on the techniques artists used in THEIR art works, what filters they use, what brushes, how they color certain scenes, etc. Yea it's not the best way to learn how to improve their artwork, but it's not my goal with art to constantly be improving, it's just having fun with what I've got.
(though i get the occasional "god my art sucks, m art from two years ago looks better what happened", that's just something every artist goes through eventually)
When it comes to comics, it's pretty much impossible to keep up a good art quality while keeping up a consistent schedule unless you have assistants. Which, most creators don't, and a lot of creators have jobs or are students themselves, and can't do comics full-time. Sometimes, you gotta trade quality to get something out.
Now to answer your question, why do some artists not feel motivated enough to not continue trying to improve: Posting something that's takes as much effort and is as time-consuming as comics in places where there are so many creators that you could post for years and still only be able to get a FRACTION of the popularity some of the biggest comics have on the platform, coupled with the fact that so many creators on Tapas are hobbyists who don't and are most likely unable to do comics full-time, can make one feel burnt-out and unmotivated.
This doesn't account for the other factors: comments (especially if you post on Webtoons, geez), the fact you're basically making your comic for free and often getting little to or no compensation, the fact that for some of us (like me), have to go out of their comfort zone and essentially turn their social medias into promotion pages for their comics because simply just posting and adding the right tags juuuust doesn't work as well anymore, and other stuff.
I remember when I was doing comics (I'm in hiatus since I'm working on a new one), I had no buffer due to some computer problems I had, and my updates would ALWAYS be late, and even when it's late, my work was sloppy, I kept forgetting to fill in spaces in the background, there are wholly uninteresting panels, the backgrounds were ridiculously plain, etc. This, coupled with the fact that I was busy with school, made me put my comic in indefinite hiatus.
So yea. A lot of creators stop because of these reasons, some just don't have the time, some just can't really conjure up the energy to dedicate their time to comics. Not really a matter of "all they have to do is improve!".