Part of the reason YouTubers have 3rd party sponsor spots is because viewer an blocking makes their Adsense revenue unreliable.

I think the primary reason for that is so they still have revenue when YouTube randomly demonetized their videos. Whether for a false detection of something that's not "advertiser-friendly", false copyright claims because someone remixed music you've licensed and uploaded it to a library to be scanned using Content ID, or if YouTube just decided to say "fuck you" and say your videos are getting Invalid Traffic for no reason. It's a diversification of revenue streams.