I think that the appeal of translating comics into animated films was a mix of three main reasons: animators didn’t need to worry about story telling, the nature and appearance of the characters had already been determined, and most importantly, comics come with a built-in audience.
As someone who dabbles in the arts, you can be really good in a few areas, but maybe not so much in some others. Hand drawn animation is already a daunting and time consuming task. If you’re animating something someone already wrote a story for, all you need to do is focus on making the character move and do the action. It’s already a lot of effort to make a character pick up and drink a glass of water; it’s also not much of a story. Why waste time drawing a character drinking a glass of water when you can focus on making them go on some wacky adventure that someone else wrote?
Having to design what multiple characters should look like is also a daunting task. Each character ought to have their own personality; if you spend the time to design each character, give them their own wardrobe and personality, and end up not using them, that just feels like wasted effort. It’s highly inefficient to do that. Why spend the time designing each character when you can use a pre-made character from a comic?
Finally, comics often have their own fans, which is super important in helping the animators turn a profit. Why waste time, effort, and money on a character that might fail? Comic characters are like celebrities. Would someone be drawn to a movie featuring Mickey Mouse or a random mouse? By making an animated feature on an established character, they are ensuring that they will earn some profit to make up for the time, effort, and money spent on animating the comic character. By featuring a comic celebrity, the animator is ensuring their success.