By the time cartoonist Matt Groening unleased The Simpsons on an unwitting world in 1987—it debuted as a segment on Fox's live-action sketch comedy The Tracey Ullman Show—the era of the theatrical animated short was long over. Movie audiences of the 1930s, '40s, and '50s were accustomed to getting cartoons before features, and studios like Disney, MGM, and Warner Bros. famously produced them by the hundreds. Characters like Mickey Mouse, Donald Duck, Bugs Bunny, and Tom & Jerry thrived in this medium for decades. This was an era we now call the Golden Age of Animation.
But animation was commonplace on television by the 1960s. Ex-MGM animators Joseph Hanna and William Barbera proved that cheap, made-for-TV 'toons could be successful, and other producers followed suit. People who wanted to see cartoons could do so at home whenever they wanted, so movie studios phased out their animated shorts over the course of that turbulent decade. From the 1970s on, theatrical cartoons were parceled out only as occasional treats, usually in conjunction with children's films. That's the way it still is today. If you see a Pixar feature, for example, you expect a short cartoon first. But you won't get one with most live-action movies.