Yes, deterrence is a factor... also very difficult to quantify reliably, so again, throw that into the ORM equation for leadership and I don't think that will prove to be persuasive.
Your point about culture change also suffers from an assumption error. The argument isn't that we couldn't make that change. It's that in the ORM calculus, it's not a change that leadership sees as necessary in the face of required resources and clear increase in risk of doing so.
This is an emotional topic - mostly because it just happened and because of where it happened. When viewed objectively though, the individual risk of being the victim of an active shooter event is so statistically small. Cold comfort to the victim's families, but policy based on emotion, rather than solid analysis of the risks is a bad way to do business.