You should do more research on these cases the TSA does stop a number of firearms but the number of people who are actually fined and or arrested is quite small. Because the TSA does take into account that carrying and transported some firearms is legal and while it is not legal to carry them past the TSA checkpoint people can forget them in their bag because it's the same bag they use when they're not flying.

A vast majority of these cases simply result in the person being questioned and then returning the firearm to be properly placed in a checked bag but not allowing them to cross the checkpoint with it.

It's not just firearms that are illegal to bring past the TSA checkpoint knives and other such "weapons" are as well. The TSA likes to tout their numbers because it makes foolish people feel safe. In reality the number of people with bad intent that they stop from bringing weapons on board is effectively zero and this is by their own admission. I myself would travel weekly and I had a multi-tool with a knife on it one week that I had forgotten was in my bag. The TSA found it pulled it out and let me on my way. Yes they could fine someone, but considering that this is legal behavior just not in this very strange area they do not push this issue unless there is egregious violations because they are not confident that they would win in court. They are confident that they could win in the fact that they can limit the area but not fines in jail time.