Look at it from the perspective of the "bad guy" you could hypothetically stop from harming innocents.

If the bad guy sees a guy open carrying at the place where the bad guy is about to cause some mayhem, assuming the bad guy isn't there simply to commit suicide by return fire, assuming the bad guy is there for some violent objective, assuming the bad guy has a functioning brain, one might expect the bad guy to shoot the open carrier first and then go take the open carry gun, increasing their own fire power and ability to accomplish their objective(s).

If the bad guy sees a guy wearing a bunch of NRA apparel or USCCA apparel or camo stuff or whatever, the bad guy might also assume that this person is concealed carrying and again might take such a person out first before further pursuit of their objective(s).

A person wearing plain clothes that don't attract attention who is concealed carrying well (no printing) is likely to blend in with the sheeple, hence the "sheepdog" moniker adopted by many concealed carriers.