So they'll spend next 30 seconds and close your website? Not good for ad profits. Ideally they should spend 10 minutes searching for something, looking at ads, then finally find it and return to this website next day.

Or maybe do a good job so that next time you'll be their first (and only) visit?

It's not like they have a monopoly, so that should be a valid strategy.

Isn't that the short term thinking that's basically killing social media sites. Like yeah we engage for hours on low quality content for now but eventually move on. Instead what would be better is consistent high quality content that keeps a user coming back day after day, even if a session is shorter. It's reliably good. Like Facebook has nothing interesting for a week even if it's find my self trapped in infinite scroll for longer. I'll eventually leave.

They have pre-roll ads and some a really memorable jingle.

ref: https://www.youtube.com/watch?v=B9TAA-x85-o&t=16s

What's in it for them? Porn sites are the last I expect to care about responsible social media practices (except privacy, maybe).