The reality is that OnlyFans wasn't the first to try this model. You have to give them credit for successfully building the business, especially with several close calls between them and government regulations.
The problem is the payment processor. How the heck do you accept adult-content related payments? That is the hardest problem to solve when it comes to these things in my book.
https://www.theverge.com/2021/8/24/22639356/onlyfans-ceo-tim...
Recently they've tried to launch OFTV to try and build up more regular (non-spicy) paid content, but it's a tiny fraction of their revenue I would imagine.