What happens when I drive more than 100m away? Does the car rely on the key being "close enough" to keep the engine powered?
This is done for obvious safety reasons. Imagine for example if a car just suddenly turned off while you were going over a major highway bridge with no shoulders, etc.
I once watched a friend's girlfriend throw his cell phone out of the car while on the freeway. Had this been his car keys I would hope that the car would not immediately power off (especially since I was in the car).