There was a TED talk by Zeynep Tufekci in 2017 (“We’re building a dystopia just to make people click on ads”) – (YouTube*: https://www.youtube.com/watch?v=iFTWM7HV2UI) that briefly talks about this:
(*I’m aware of the irony in linking there)

So in 2016, I attended rallies of then-candidate Donald Trump to study as a scholar the movement supporting him. I study social movements, so I was studying it, too. And then I wanted to write something about one of his rallies, so I watched it a few times on YouTube. YouTube started recommending to me and autoplaying to me white supremacist videos in increasing order of extremism. If I watched one, it served up one even more extreme and autoplayed that one, too. If you watch Hillary Clinton or Bernie Sanders content, YouTube recommends and autoplays conspiracy left, and it goes downhill from there.

Well, you might be thinking, this is politics, but it’s not. This isn’t about politics. This is just the algorithm figuring out human behavior. I once watched a video about vegetarianism on YouTube and YouTube recommended and autoplayed a video about being vegan. It’s like you’re never hardcore enough for YouTube.

So what’s going on? Now, YouTube’s algorithm is proprietary, but here’s what I think is going on. The algorithm has figured out that if you can entice people into thinking that you can show them something more hardcore, they’re more likely to stay on the site watching video after video going down that rabbit hole while Google serves them ads.

These days it might also be about politics, but the motivation to capture attention to serve ads is still the priority.