this is really the crux of your question. gun violence in America is a multifaceted problem, but if your source of information is headline news, you will get 0 meaningful information.
> Do thieves and robbers get shot on a regular basis
this depends drastically on the local/state laws. San Francisco, for example, does not issue permits to carry concealed firearms and openly carrying firearms is illegal. California is simply a very gun unfriendly state, so you will almost never see a citizen defend themselves from a robber with a gun there. Even if you can carry a gun somewhere else, as a sibling goes into more detail, there are various laws with wildly different requirements to be able to employ lethal force. They range from requiring you to attempt to run away first to allowing lethal violence in defense of your property (like a vehicle with cameras in it).
for what its worth, gun violence in america has little to do with so-called mass shootings that make the headlines or the hated "assault rifles". the story of gun violence in america is something like 90% pistol shootings in economically damaged, inner city slums.