An episode of SB takes very little time to make, is aimed squarely at prepubescent children (though can and does have an audience beyond that), and features the same cast of characters (in general) in every episode. An animated feature takes between two and four years to complete, goes through a much longer process of storyboarding, scriptwriting, musical composition and marketing, and aims to appeal to a much wider variety of people. Just because most of these people are presumed to be children doesn't mean they all are. Think about how many Disney, Pixar, DreamWorks, Blue Sky, Illumination and Studio Ghibli films exist. There will be, at the very least, one in there that is liked by people older than 10. Hell, even SB has a bit of a cult following among some grown men and women.
Also, animated films are respected as art pieces. Beauty and the Beast and Up were both nominated for Best Picture at the Oscars! WALL-E was named Best Movie of the 2000s by Time magazine! And Frozen currently holds an 89% rating on Rotten Tomatoes! It's not just Disney and Pixar though - Studio Ghibli, creators of such classics as Spirited Away, Howl's Moving Castle and Princess Mononoke, is the absolute critical darling of the animation world. Their films are praised for maturity, artistic merit, and timelessness.
I'm not saying that any of these things can't be present in short-form animation. Far from it. Watch any of the Pixar shorts, the Wallace and Gromit shorts, the film 'The Snowman', or this little gem from Disney called 'The Little Match Girl':
http://www.youtube.com/watch?v=yUSzQBaWq0Q . But guess what? These aren't classed as 'kids' cartoons' either! Hell, I know a lot of people who won't even show The Snowman to little kids.
Do I hate children's cartoons? No. Will I actively choose to watch Looney Tunes or Phineas and Ferb if they're on TV? Of course. We all need a laugh. Do I think that there aren't animated films aimed solely at the youngest of children, with nothing to offer besides that? Absolutely. But do I think that excellent animated films like WALL-E or Frozen deserve to be lumped into the 'children's' category, thereby discouraging adults to see them? No way.