Because you'll get shot?
Kick them really loving hard and take off.
As long as you aren't slow-poke mcgee or far from people you should be able to get to people before they get up, even when they get up they'll still be in pain.
If they had a gun im pretty sure they should hold out the gun to him and tell him to drop it
And this, if they have a gun they'll make sure you know. No matter what, if they have a gun you hand over your stuff, don't try to be Dirty Harry either and pull out your own gun if you have one.