Why do we have hostage negotiators

I'm just sitting here wondering why hostage negotiators exist. If the solution to 100% prevention of gun violence is to have more guns on the scene (according to the 45th president), and SWAT already has guns, why do we have hostage negotiators? The lives of a handful of adults at a store or a bank aren't worth more than hundreds of children in a school. Do those people in the midst of committing a crime not know that cops and SWAT have guns? Just knowing it is supposed to prevent the crime. Haven't armored trucks, train cars, and stage coaches had armed guards for hundreds of years now? Has that been a full proof deterrent? Is money worth as much as the lives of children? Castles have traditionally been well armed since the beginning. Has no royal ever lost their life while in their castle surronded by armed guards? Or have many kingdoms been overthrown?

There must be some proof, in all of human history, that increasing weapons prevents crime at an astronomically high rate. Some deterrent must have been full proof, right?

Just pondering.