Steam takes 30%. Apple, Google Play take 30%. Physical stores can eat 60% unless you're selling something that's demonstrably saleable. Those sorts of markups aren't at all weird for access to a market... And that's what it is, as SatvikBeri has already said. People typically don't go to Patreon to find new content. They consume the content elsewhere (YouTube, Facebook, etc) and then go to Patreon. Facebook et al are…
Patreon not only collects money, it also provides a way to assign rewards, define goals and notify patrons (filtered by reward level) of new content.
It's not a publishing platform, but it's not just a payment processor, it's also a kind of marketing platform.
That said, the most appalling part of Facebook's "competitor" is not the 30% rate, it's that their ToS try to pull the same bullshit stunt they try to pull on their social platform: tricking you into believing you owe them perpetual transferable rights to everything you share via their platform. They know this isn't actually enforceable (at least not in all jurisdictions internationally) but they still try to trick people into believing they unknowingly agreed to it and are comfortable exploiting those in jurisdictions where this nonsense works and those who don't know any better.