Here in Miami, the news often shows that some "predator" gets thwarted by a check with a baseball bat, or a pistol, which suggests that they are not all that weak.
Most thieves are pretty incompetent losers, and most heists are poorly planned. They don't bother to check on the weakness or strength of the victims. I suppose that they attempt to rob banks less because of the obvious presence of a guard, but here in Miami, that does not seem to stop at least some of them.
Mainly, they appear to plan their Targets based on the presence of a cash register or other bunch of readily accessible money.Most are on drugs.