Well, the general answer is that charging the clientfor the product is not the only profit model nowadays. Think of things like the many Google services that are not charged and still help them be one of the biggest companies in the world. Or if that is too large a paradigm to be compared, think of something much smaller, like Discord.
I don't know those websites, since I don't even watch porn, so no idea what their specific model may be, which I get is the most direct aspect of your question, but what I am saying is the framing of, 'how is there money if they don't charge the customer?' does not really apply to present reality.
If hypothesis have any worth, it can be interesting to consider money-laundery for example, which is behind many, many business around the world. Or, if you look at how huge the survey and behavior analysis business is online, both by universities and commercial companies, which spend a lot of money on general sites like Mechanical Turk, or specialized sites for that purpose. Obviously a porn site would be a goldmine for behavior research, and instead of paying the users with money, like the sites I mentioned, they offer the videos.
This could even be of interest for the porn industry which would explain why copyrighted material is not taken down. I even remember, back when I was still into porn and watched camgirls, if you put long recordings of them on a free site, it would most often be taken down immediately by DMCA, while you could find obscure websites that had like all recordings of all shows put up there for free in a massive scale. And also other sites that charged for a subscription, outside of the official camming site, and served millions of recordings without paying the models back.
In this case make it seems very suspicious that the main cam site, like Chaturbate, or Cam4 or whatever were either behind or connected to those shady, technically illegal sites, since they would DMCA stuff off PornHub in hours, but those sites with a full assortment of their content managed to keep it, and sometimes even chage for it.
TLDR, profit models online nowadays greatly transcend having to chsrge your customer for a product, and porn is a shady business by definition, so it can be full of background motivations connected to it,