Because the Simpsons have purposefully crude and simple designs, and the more you draw things in a way where it feels more raw and organic than pure geometry, it just fits. That's why I even like the way characters are drawn in Season 1 (most of Season 1, anyhow, some episodes have background and crowd shots that look ROUGH).
I think the show peaked with its fun, loose look from Seasons 2 through 5. After that things gradually became more and more stiff, but in recent years we're getting some nice moments of animation again, like Mr. Burns hallucination from drinking Cletus' alcohol, but those are few and far between.
You don't really get too many shots like this anymore.https://www.youtube.com/watch?v=Dfsi2CHS8Bc#t=1m
Go to the one minute mark and just look at how the humor is enhanced with the low angle of Marge talking about the bird feeder, and then the high angle that looks down on the kids while they express their silent disbelief.