It's simple physics. Stopping distance increases with the square of an increase in speed. Double your speed, and it takes twice as long to stop while you're covering twice as much ground, for four times the stopping distance. *
For example:
I've got a hill or two which I'll climb at 3 or 4 mph loaded down with too many groceries, and a car door isn't a surprise at that speed. At those speeds, I really could look inside a parked car and count the loose change under the seat before an opening door is a danger. On the other hand, going downhill at 30 mph I need 100 times as much distance to stop, and from that far away I really would need x-ray vision to see what's going on inside a parked car, and esp besides to know when to hit the brakes that far in advance!


* Yes, we know you Googled it and found out that this is only one part of how braking distance works. The explanation overlooks decision and reaction time. But that's how this works, you asked a simple question and you got a simple answer.