The membership fee is $99/yr. That's the equivalent of $330/yr in App Store sales per developer.
The opportunity cost of having developers NOT sign up because of the fee seems negligible. People still come flocking to Apple because they think they need to build an iOS app (and because of Apple's lackluster adoption of progressive web app support, that's still the only good way to get iOS users on board).
EDIT: Also, as others have mentioned, having a lower or zero fee would lower the bar, potentially flooding the App Store with even more low quality apps.
The opportunity cost of having developers NOT sign up because of the fee seems negligible. People still come flocking to Apple because they think they need to build an iOS app (and because of Apple's lackluster adoption of progressive web app support, that's still the only good way to get iOS users on board).
EDIT: Also, as others have mentioned, having a lower or zero fee would lower the bar, potentially flooding the App Store with even more low quality apps.