Because a lot of times you want to actually use the new thing for a while to see if it works for you. But with no way to go back, it's a one way door, so you better have access to someone else's Tesla if you want to just try it.

I do own a Tesla.

You're not forced to. The car will alert you when there's new software available and even download it when you're connected to wifi (which I never am because I live in an apt and park far from my actual apt), but you still need to decide when to schedule the install, because it can take 30-90 minutes to complete. The UX has not yet degraded to the point where Tesla decides 'eh, you don't need your car at 3am to go to the airport/hospital today.'

I do believe there are cases with critical bugs where Tesla can force the update.

I have the v11 update sitting on my car and I'm not installing it until I hear some of this stuff is fixed.

It nags you incredibly. Everytime you sit in the car, the pop-up is there, nagging to install now, or to schedule install overnight. The way out of the pop-up is a small [x] to close it. And the dialog is right back the next time, waiting until you (or somebody) will accidentally press the other button.

I have it sitting on that screen, nagging me, and, honestly, it's depressing.

The problem is that it is all or nothing. You can't accept each new feature separately only all of them at once.