Some can be, some the manufacturer doesn’t want to risk it so they make you take it into a dealership to update from a USB.
A Tesla always updates over the air (I suppose unless that’s the part that’s broken). It’s arguably the most important safety feature on a car mostly defined by its software. I have a ten year old chevy that needs a software update, but like you said I’ll need to make an appointment to have someone else download it and manually install that software for me, which sounds super archaic and dumb when it’s spelled out like that.
Maybe they don’t have an immutable backup firmware and are worried about bricking some part of the car if the update fails, or it’s a hold over from their old car recall process.