There is a good, logical reason for that.
The attention of the armed robber is all over the place. They have to watch their victims, watch for police and onlookers, watch for cameras often enough, etc. Their intent is to get stuff and escape, he wants to temporarily control, usually not to kill.
This is why there is a huge advantage to a concealed armed defender. His focus is narrow, just the robber and his weapon, and for the defender to recover his weapon and shoot the attacker. He doesn’t have to fret about police, anyone behind him, onlookers, taking stuff, or escaping. He does not want or need to dominate or control the situation, just to put rounds through the robber without getting shot himself.
This is why it is much easier for the unarmed defender to snatch the robber’s gun. For most robbers, if their victim pulls a gun, it adds so many variables that they prefer to flee.
Yet in that situation, the defender still has advantages, because he knows “the terrain”, and still has his limited focus. The robber is messed up. His robbery is a failure and he will probably not get what he wanted to steal, his life is at risk, he has to assume cops are on the way, and even if he shoots the defender or anyone else, he could face a murder rap, not just armed robbery.
So fleeing is a very good option.