I have no knowledge as to why but if all/most fixed wings have seperate ignition circuits perhaps it hearkens back to removing electrical power from a piston engine propellor free wheeling in the slipstream or turbines with fixed power sections? Do turbine aeroplanes have freewheeling units?
I’m certainly glad they don’t have that system on the helicopter PT6. Opening and closing the throttle then modulating and using the starter and also activating the idle detent is already a bit of a dance of the hands.
It is an interesting question.