Because sane OSes have sane defaults, and this is one of them. Hide information by default, unless called for. Want verbose boot log? Ask for it. Want boot menu? Ask for it. Need Bluetooth enabled at boot? Aak for it. Don't overburden the user with irrelevant info. When my 7 y.o. daughter fires up the Steam Decj, she doesn't need to see the boot menu.
No, they don't; they let you decide some things which are relevant. People in the know decide defaults. This is how it works everywhere. Else you'd learn to bicycle with your own traffic rules. You'd end under a bus.
Addendum: You could say you want a sandbox, and that such is part of the sandbox to play with. Then you need some kind of way to clean up, like you can clean up the toys your kids play with (or ensure they clean up their own mess), or ensure their sandbox environment is safe (such as no fire hazards in your house). Then I would argue a VM is such. OS with rollbacks or a user account on an OS or locked down iOS/Android could suffice, too.
My parents gave me the bicycle in a box. I had to put it together if I wanted to ride it. They owned the bicycle shop too. They could have put it together for me, but they let me do it.
