Interestingly, I classify FMA as fantasy, as it deals with "soul power," and the trained fighters' physical prowess seems to exceed that of trained fighters in real life. But its lack of spaceships is probably a factor for my subconscious, too. XD I would say it's an example where the line between fantasy and sci-fi is blurred.
FMA has plenty of science in it, for sure, and it's all very logical/internally consistent... but so does Death Note, which is CLEARLY fantasy even though internal logic is just as crucial for that story.
I don't think the ease of drawing is a big factor. It could be, but it doesn't seem that way? Chobits is sci fi. Also, steampunk is probably just as much pain to draw as spaceships, but it seems more common than spaceships? I would argue pretty much everything is hard to draw. My observation is that when someone wants to do a comic but doesn't feel confident in drawing something, they tend to just skimp on it, which is why the background is often absent or just barely there in many webcomics.