It's also because scripts are a form of book. They're a medium of their own and what feels good in the medium of print often doesn't feel good in animation. As an example, scripts tend to be dialogue-heavy, even when they're written by artists. That's because ddialogue driven scripts are leaner and easier to read. Dialogue comes in trim little columns surrounded by oceans of white space. It looks better on a page. You can read it faster. It's an amazing but true fact that dull, dialogue-heavy, talking head cartoons get made for the trivial reason that their kind of script is easier to read.
Here's an example. This is an excerpt from a first-draught script I wrote for Animaniacs. A witch's candy-covered house attracts the Animaniacs and she tries to eat them. They turn it around and harass the witch to distraction. The script reads OK whenever it depicts dialogue but watch how hard it becomes to read when it describes visual gags:
Which part would you rather read?
It's also true that stories that originate on storyboards tend to emphasize visual gags, the thing that animation is best at. When I'm drawing I naturally pay more attention to the way a character looks in clothes, the way he bends to pick things up, etc. Sometimes these details are so funny that I end up building a whole sequence around them. That feels right to me. Comedy is best when it's about little things. Scripts, on the other hand, favor the overview, the big things and the complex subplots.
Now that scripts dominate there are very few funny cartoons. Since scripts are uncongenial to visual comedy the powers that be have decided to eliminate visual comedy. This is the shocking price we've had to pay for our script addiction.