Apple take 30%. You have to understand that you're not paying just for the service, but the reach. You can DIY your donation platform and the tech side isn't rocket science, the issue that platforms like Patreon and Apple have access to orders of magnitude more users than you can ever generate on your own.
The couple apps and podcasts I've subscribed to that did roll their own payments and subscriptions have been hell to cancel or adjust my subscription, riddled with dark patterns, and one seemingly just... removed the unsubscribe button entirely. There was no way to cancel at all.