Surely a developer or ten has suggested the same thing by now?

My guess is focus: they are 100% focused on being the best closed platform that is easy to use and everything is handled for the user. A lively sideloading and 3rd party homebrew scene would be a massive distraction for the Dev team and most importantly the support side with little direct business ROI (indirect could be big but a harder sell).

Possibly harming brand reputation showing people what they could have absent the paid wall. When the alternative exposes people to risk, and they'd rather not have to explain themselves?