It's not that simple. Porn is a huge risk for a company, especially if it's user-generated content. You have to police access against underage kids who want to watch that content, and also have an enormous team devoted to immediately take down CSAM.

Lots of payment processors and advertisers do not want to associate themselves with porn because there are too many PR risks. Ask Pornhub how difficult it is to be a porn company on the internet, they've had lots of thorny problems. Or look at Reddit and how it's handling porn content before the IPO: they can't outright ban it or they'll lose users (e.g. Tumblr, another great example of what I'm talking about), so they are trying to hide it as much as possible.

Basically trying to monetize porn is not worth the headache. Last but not least, because there is so much free porn around. Risking so much to gain a few users that could go somewhere else in a whim is REALLY not worth it

It's not like they don't have any age restrictions, the streamers that do that stuff have to mark their streams 18+ and add 'sexual themes' tags or they get banned. If chatters even imply their under 18 they must be banned or the streamer gets banned. Im pretty sure you cant see 18+ streams unless you tell the website you're 18, which would mean it's about as much of a gate that pornhub makes you jump through. They make gambling streams have the same rules but their tag says may contain gambling.