Developer programs are supposed to be a loss offset by the higher amount of sales of your devices due to having more apps thanks to the developer programs.
Paypal, Stripe, 2checkout etc. process transactions for 2-5.5% fees, not 30% (of course 2-5.5% is also extortionate, but that's mostly due to the VISA/MasterCard duopoly, also very evil, and the horrible idea of credit card chargebacks).
Bandwidth is 0.10$/GB, so it would be another 1% at most for most apps, and doubling and rounding would make 15% the very maximum acceptable cut, with 5-10% a more reasonable one.
But of course most developers are themselves complicit in exploiting the userbase with obscenely pay-to-win games, trivial apps that are $1-2, in-app purchases and more, so they are basically content to share their loot with Apple in exchange for Apple's approval of their dubious practices and protection against their users' attempts to not pay.