It's a safety issue.
Crazy inexplicable things can happen when modern cars have electrical control access to brakes/steering/throttle.
Firmware doesn't take right, download message emulates by accident another message, random bit flipping that even CRC checks miss.
There's no reason to risk any of that. Just shut the engine off.
Because it's a main source of active power that can be re-directed.
A battery on an EV can be too, in which case the main contactor should be verified disconnected prior to any software update. But typically people don't refer to that as the 'engine'.
So prime mover encompasses engines and main batteries.