There's some thoughtful comments here already, but I wonder the same thing constantly as a fairly addicted user of YouTube who wants to avoid short form video altogether.

I think Premium users tend to be the most affluent desirable group for ad targeting (similar to iOS users on other platforms) and even though YT Premium lets you avoid ads on YouTube, I suspect one's activity feed/"algorithm" on YouTube factors a lot into Google (and others'?) ad targeting. The same eerily effective feedback loop for getting TikTok and YouTube suggestions works better with short-form video, so even if users aren't seeing ads, YouTube still has an incentive to have people use it. So, there's money to be made in dialing in your "algorithm" from using YT Shorts even if you're a premium user.

I'm sure the other stuff about KPIs for increasing usage of shorts to compete with other media sites is accurate too