1. Subscriptions are easily monitored from a single page.
2. Before subscriptions renew I get emailed, not all services do this, shame on those that do not and a magic renewal transaction goes through without notifying you first
3. Trusting a single entity (Apple) with payment is easier for me to stomach than giving my credit card info to a dozen or more different sites/services manually
4. The user experience is vastly better using Apple's solution than having to enter all of my billing info by hand each time or with Bitwarden even which can get things wrong often enough I always have to double check.
As a developer you gain the advantage of users being able to easily purchase, and users have a credit card assigned to their Apple ID making purchases incredibly easy. I'd argue, without proof anyway, that this probably results in purchases you'd never get otherwise due to how easy and seamless it is.
The 30% cut is a lot, but I think in general there are big benefits for users, maybe the developers don't see it that way, but as a user I would NEVER use something other than Apple's payment solution in the App Store. So if you're asking for that option and I seen you were charging more for it through Apple's solution and less through some other service I'd immediately not pay for your app. Simple as that. You'd basically lose me as a potential customer.
Edit: because apparently line breaks are fun