An underrated question. The answer of course is twofold:

* Paternalism (Apple believes users are too stupid to be trusted to control their devices, and no amount of "I really know what I'm doing" confirmations could change that)

* Apple's biggest fear is being disintermediated by Google or Meta, the way WeChat did to phone manufacturers in China. An ability to side-load an app could allow a foothold for a powerful competitor who could wean you off of all their lock-in features in favor of an experience that would tie into a competing ecosystem.