Interesting question. The answer is because people pay to have quasisocial relationships with the OnlyFans providers. On other sites with just videos, you don't get that same feeling.

Basically, people are lonely and will pay to feel seen, heard, or loved.

If anything sites like PH seem to be advertising venues for things like OnlyFans, cam sites and even Snapchat profiles.

Is your argument that there is enough free random porn to make it not worth paying for OnlyFans?

The reason I would pay for OnlyFans content, if I could afford it, is that there are specific creators on there that I find particularly attractive, either physically or personality-wise, and I would love to help fund their lifestyle.