It goes way beyond the Seat Belt Nanny.
Have you tried backing up a new car with the driver's door cracked open? Several won't allow it – refusing to engage Reverse until you close the door. You can try all day to move the shifter into Reverse – but programming controls whether the transmission will comply. It's all drive-by-wire now, you see. So no direct, mechanical connection between the gear selector and the transmission. When you select a gear, you are merely asking the computer to engage Drive or Reverse – and the computer is the ultimate Decider, not you.
But why would you want to back the car up with the door open? Not that it is anyone's business – certainly not an insolent computer's – but that's how people used to be taught to Reverse in close quarters, because you could see better. The curb, for example. Of course, this entailed some judgment and motor skills which are now lacking – and certainly not taught. Instead, people are taught to gape at a screen rather than actually look in the direction they're backing the car up.