Fwiw they're just collecting a paycheck. Sure it's scummy for them to not reject the businesses offering them the money but tbh I really don't blame them - I'd probably take the cash too.
Depending on the channel size sponsored ad reads can deliver upwards of multiple thousand per video for the creator. If you see multiple channels with the same ads, it's bc the company advertising got a big budget approved, that's it.
Imo just skip the ad reads (or get sponsorblock) and forget they exist. Usually the creator doesn't even give a fuck who's paying them or why. They are victims of the system too, not maliciously peddling garbage. I do wish they didn't have to peddle anything, but here we are.
I think there are many others one could point to more accurately, including Yahoo who purchased and then resold tumblr.
While Tumblr has made a lot of changes, some of which were walked back, they are still a well used and beloved site today. There are many now-dead or barely alive behemoth Internet companies that are better examples of "poster child for running a terrible Internet business" imo.