Firstly, to make them cuter... like babies. All baby humans have proportionally big heads/eyes to the rest of their bodies and we're hardwired to think of those characteristics as simpler, cuter, and deserving of our attention.
Secondly, to give the animator a face that is both simpler and larger which makes it much easier to draw expressions
These two things make it much easier for the audience to relate to the animated character and for the artist to draw it