Because it's a comedy. The answer to all these questions is that it's a comedy.
It's less detailed because details often distract from the point of visual gags, the proportions are less realistic because small eyes and small heads don't express well and expression is essential to visual humor, and so on and so forth
Don't get me wrong, I don't think it looks good; on the contrary, it's bland as fuck. But I feel like /co/ doesn't really get WHY it's bad.
Comparing it to the previous incarnations isn't at all fair because it's a fundamentally different show. Instead, we should be looking at it next to older comedy shows like Ed Ed and Eddy and Spongebob, both of which did what it does far better