Lumper, aside from plain fear, there's a relatively simple answer, IMHO, for that question:
Because, since the time they were little kids they've been conditioned not to fight back, not to defend themselves, not to be "confrontational", had mandatory "conflict resolution" courses and all sorts of liberal bullsh*t indoctrination aimed at making sure no one gets offended.... need I go on?
They've been taught to lock down, shelter in place, and wait for the police. Except in this case, the police were too busy peeing down their own legs outside to do the ONE job they're supposed to do - "Serve and protect".
Besides, charging a guy wielding a gun is not something most people would do anyway - especially when unarmed. I suppose it would be a different story if someone or others was able to return fire and keep the bastard's head down, to allow others to work around behind him (IF they even could in the narrow confines) and THEN take him out, but that just wasn't the case.
I don't blame the kids at all. The immediate fault lies completely with the "resource officer" and Broward deputies who cowered outside while this psychopath was allowed to roam and shoot.
The next larger picture is the system which allows these stupid "Gun Free Zones" and the rest of us who just sit by and allow it to BE that way.
The 30,000 foot view above that is the fact that this kid was KNOWN psychopath, who SHOULD have been put into the No-Buy NICS list immediately after being expelled for making terroristic threats and +45 LEO visits to his home for domestic violence. NOT ONE ALPHABET AGENCY DID IT'S JOB, and that fact and that fact ALONE allowed this little asshole to buy firearms.
We have to send our kids to these places by law, and they're defenseless there, by law.
The local cops responded quickly, and described it as "blowing past the Broward deputies outside".
Those local LEOs deserve all of the credit for response and for their bravery.
The Broward personnel that failed these kids and their little fascist wannabe Sheriff - you know, the guy who quotes Game of Thrones saying "the lion doesn't care about the opinions of sheep" - need to be removed from any position of authority ASAP.
And the ONLY thing that needs to be fixed regarding "gun control" is that the assholes who are in charge of doing a job actually DO THEIR JOB when it comes to making sure people like this aren't able to legally buy a firearm.
The other reason they never try to disarm the gunman is that they are terrified of guns, and once a gun is in someone’s hands, that whole person becomes a huge gun, in their minds. So when he stops to reload, they just cringe and cower, because to them, the gun is magic, and he who holds it becomes as powerful as they think that gun is, even when the gun is temporarily unusable. That’s my theory, anyway.