Honestly, it's probably because 3D animation isn't made for the squash and stretching traditional animation can bring since they're models being rigged. And traditional animations actually benefits from imperfection.
The best examples I can think of come from the old Simpsons (done cel-shaded) compared to the new Simpsons. And Sailor Moon.



old vs new.
Notice how stiff the new looks? With the pacing deliberately made so that it saves as few animation frames as possible? This is the problem of the modern. Everything looks prettier and cleaner, but at the expense of the actual animation and character.
The allowance to go off-model (and animate fully) makes for more dynamic poses which we can understand and relate to because of the extremity. It doesn't matter if the inbetweens are ugly or distorted because the final animation puts the point across. The newer one, while cleaner to look at, has stiffer proportions. But it makes prettier gifs.


Do you believe Belle's disgust more over Elsa's laugh? They each have the appropriate moving parts, but does one seem stiffer than the other? Elsa sure looks prettier and cleaner, you can tell she's "giggling", but there's also a definite uncannyness about it. Without any imperfections due to the natural human nature of animation (lines wiggle, pencil scratches show up for split seconds, the body distorts), it comes off as stiff because it's almost
too perfect.
Of course, some people aren't bothered by this in the slightest, but if you'd wondering why you feel 3D has this fake plasticity to it, it could be because you're noticing it's without imperfections, dynamic expressions/poses, and the rigging has confined it into never exceeding itself lest it break. Whereas traditional animation...
I doubt we'll every see the extreme poses Gaston has made again, but you sure do feel them sometimes. Just like the concept art of 3D films. You feel and relate to them more because it's imperfect but also dynamic. But as
@cactus has pointed out, it's way cheaper to do 3D films than traditional animation. Despite the introduction of the Cintiq, you still need some talent to be able to keep your characters somewhat on model, lest you end up with the Steven Universe problem.
3D you can copy the rigging on the model for multiple animators to work on, so it's not as big of a deal. So why pay for traditional animation anymore when you can get some college level kids to do the work and have the budget be significantly less for a faster product. After all, this is the age of Consoom. Just Consoom movie and movie products and get excited for next movie project by Company. Instead of waiting years between projects because animation takes time.
