Developers only put up with the curated app store because it's the only option. If there was an alternative, major companies would mostly choose to bypass the app store, leaving it a barren wasteland like the Mac app store.

If Apple allowed sideloading but went out of their way to make it obnoxious enough that everything which could be would still go through the app store, you'd just have all of the exact same complaints about how Apple is deliberately making things painful for users.