And don't call them cartoon movies, either . Calling an animated feature a "cartoon movie" is like calling Annie Hall a "sitcom movie" or Sunset Boulevard a "soap opera movie." There's nothing wrong with cartoons, just as there's nothing wrong with sitcoms and soap operas, but a true animated feature is a whole different animal.
(That said, I have seen a few gag-oriented animated features in recent years that resemble cartoon shorts stretched to feature length.
Also, I saw an episode of the old Disneyland TV show - I think it was the one on Lady and the Tramp - where Walt and the animators referred to them as "cartoon pictures" or something like that. But that's okay, because you know they had respect for the medium .)
The mainstream media refers to cartoons as "kids' movies" because the mainstream culture considers most animation to be child-oriented. They did back in Walt's day, too. Animation has been associated with children for decades. Sure, in the 1930s and 1940s there was a certain effort on the part of animators to appeal to MORE than just children, but animation and children have gone together like peanut butter and jelly for a very long time in this society.
Remember the 1972 Ralph Bakshi animated film Fritz the Cat? The marketing for that film made it abundantly clear to everyone that it was NOT for kids, because, based on its character design, one might think it was intended for children.
Most of the adults who recognize that animation is not (or at least should not be) limited just to children and/or teens are usually already aficionados of the medium. But the overwhelming majority of animated films made today and in the last century have been made with either children as the primary audience, or inclusive of children as a primary portion of the audience.
Most people who watch The Simpsons, Family Guy, and American Dad know that animation isn't only for kids, though. It's just that most is.