The fact is not that I do not love grain, at the contrary, I DO, and a lot!
But, to me, film should have it, while animation, even if, as you wrote, "lived" on film, should not, for the simple fact that grain in film was part of it - and film without grain, fine or coarse, could not exist (at least, "at the time" of film), and the director of photography used it to give the film the final look that could be seen in the theater, while grain in animation film was a "consequence" of the process...
For example, what if Disney in person had the chance to choose to release his animated features with or without grain?
Today, in our digital world, there are few, great directors that could still choose to actually "film" their movies, while some others are forced by studios to use digital cameras for mere economic reasons; and many others apply grain to digital sources to make them more "filmic" - this is why grain plates are gaining success today.
Not the same (IMHO) for animation: there is no need to give life to a lifeless source (beware, in this context intended as not live action!) - but, again, this is just my personal POV.
Last thing: even a century ago, it was possible to project animation on film without grain, just drawing it on blank film:
of course, as it was a really time consuming process, it was not widely used, but still possible to obtain grain free animation before CGI!