Games require engagement. You can't get up from your desk or couch while still playing. You can't fall asleep while actually playing. So compared to video, Google can sell more valuable ads on games in YouTube because they can guarantee to advertisers that "this person has their eyes glued to the screen, hands on mouse or controller, and their butt in the seat."

Shorts is a less powerful version of the same idea. It's also why long-form content is falling out of favor with the algorithm.