Ask HN: Why bother sponsoring YouTube creators? Why not just pay YouTube?

Reformulating your question: Why sponsor a football team when you can just buy ads on TV?

Advertising and Marketing is (mostly) a way of making your brand more visible to possible customers. Sponsoring a trendy youtuber you get more visibility than other companies that just bought some ads.

You also have to take into account the friendliness of your advertising. Some ads might become annoying just because you saw them a few times in a row... sponsoring allow brands to buy you a "friend" that tells you how great they are.

Giving enthusiasts products lends enthusiasm to it, at the risk of a bad review. Advertising beside enthusiasts only makes products visible (to those who don't block ads).

Maybe they think they've run out of new enthusiasts with sizable audiences, so follow up by supplementing with ads to remind viewers of what they saw before.

Ad blockers don't block the ad if it is incorporated as part of the video.