Because the videos in YouTube premium still have ads. But because the ads are built-in to the videos by the "content creators" themselves, they call them "sponsor segments". YouTube Premium doesn't include SponsorBlock.
It does in a limited capacity now [1]. No, it doesn't auto-skip for you and it's not nearly as quickly updated as SponsorBlock itself is, but it works fine enough on the iOS app.
[1] - https://9to5google.com/2024/05/05/youtube-premium-jump-ahead...