Probably because the majority of cartoons are aimed at children, while only a few are geared toward older teens and adults.

Animated cartoons were originally part of the short subjects which were shown in movie theaters along with whatever movie was the main feature. Almost all children took delight in the cartoons, but not all adults enjoyed them and that fact created the perception that cartoons were for children. Even as movies entered the sound era, there were exceptions such as most of the fare from Fleischer Brothers studios which was aimed at adults but which could also entertain children, and cartoons from Warner Brothers which was aimed at both children and adults. The exceptions were overlooked, as exceptions usually are. When television started coming into its own after world war II and more and more households were buying their first television set, old cartoons were easy to broadcast and were popular with children, but not with most adults. When original cartoons started being made for television, most of them were relegated to either afternoons after public schools had let out or saturday mornings, time slots that were essentially ghettos for children's programs which also helped booster the idea that cartoons were mainly, or only, for children. A few cartoon series (The Huckleberry Hound Show, The Flintstones, Bullwinkle, and The Jetsons) were broadcast in prime time and had large audiences but those exceptions did not last many seasons, while the saturday morning cartoon ghettos of children's programs continued on unabbated for decades.

What is probably more interesting than the idea that cartoons are for children has been so persistant is the idea that some cartoons are for adults managed to catch on. I think that the exceptions to the idea that cartoons were for children helped pave the way for fare that genuinely was aimed at adults. Certainly the first anime series that were telecast in the U.S.A. were aimed at children and it wasn't until the cable revolution introduced the concept of "narrowcasting" and most households had more than one television set that it was possible for cartoons that weren't aimed at children to find a target audience in North America.