The way our constitution works, a more correct question to ask is, “why don’t they think it’s a bad idea?”
In the US, you are free to do absolutely anything you want unless the “people”, through their elected officials, think that freedom is a bad idea. e.g. murder, theft, etc.
To the correct question is, “why should people be prevented from carrying guns in hospitals?”
“They” have had Carte Blanche for too long.