Yes, I get that you can switch to Android and Apple doesn't have an actual monopoly. They still wield a lot of power to force this 30% tax down everyone's throats. Specifically it's the power of "bundling" which is a potential antitrust violation.[1]
The implied reasoning behind Apple's restrictions on bypassing the tax is to ensure a positive customer experience. By selling things through Apple they take on the role of an intermediary merchant that can do things like refunds and otherwise ensure a positive, trustworthy customer experience.
But we need only look at the Mac App Store to get an idea of how much consumers truly value this value-add. On that platform, vendors and consumers have the freedom to directly interact, while Apple has still enacted a pretty good level of privacy and security protections (through revokable developer certificates and sandboxing). Many simply bypass the store. And the tax there would make even less sense for strictly ongoing content subscriptions that pose no additional security or privacy threat.