Probably because standing your ground is a much less effective tactic than retreat, in nearly all situations.

The only time I've seen someone with a gun get the upper hand on someone else with a gun is by first fooling the armed assailant into thinking they're unarmed, in order to gain an element of surprise. Invariably they're calm and collected bad asses who look like they just stepped out of an 80's action movie. While the asailant is looking the other way, they casually reach for their gun from under their shirt and blow the assailant away all in one smooth motion. Look up "undercover cop in Brazil" and you will find half a dozen such videos. Unfortunately 99% are not badasses.