The issue with the browser (and sideloading) is security. Browsers by their nature are essentially apps that run arbitrary code from an unknown location. How do you ensure security of the devices if you don’t control the browser?

Safari is great on iOS. I’ve never felt the need to run something different. Same with sideloading apps. I’ve never seen the need for that. Maybe I’m an Apple fanboy but I think they’re doing the right thing in both cases.