updated My 29. 2023
Most casino security guards used to carry guns. For a period, very few guards remained armed. But the trend has reversed again and nowadays, some guards carry holstered handguns.
The main reason for the disarming of security was and is, as you might imagine, money -- money saved from having to shell out in settlements for accidental shootings, money saved from the cost of the training required for each and every guard, and money saved from the high premiums charged by insurance companies to allow casino security to carry heat.
Insurance? We've heard of instances where security guards have misplaced their weapons, dropped them in the casino pool, pulled them in beefs with fellow officers, and discharged them negligently (one guard accidentally shot himself in the leg and another committed suicide with his weapon). So you can see why the insurance premiums run high.
In addition, the guns are usually the property of the casino and/or they have to store the weapons on site (this raises the insurance cost even higher and lowers the safety of the environment).
Another thing about the casino's money is the casino's money: It's insured. For max value. After insurance settlements, a casino never lost a dollar to a robbery. So, given the potential for harm and lawsuits, the casino has little incentive to arm its guards to prevent getting ripped off.
Finally, the whole purpose of carrying a sidearm is negated when the security officers are told to never ever use or even present their weapon. Should they unholster a gun, they're completely on their own.
For example, if an officer tries to help an elderly woman being attacked by using deadly force, he can be sued, and even prosecuted, as a private citizen.
All in all, any losses a casino sustains from a criminal action that an armed security guard might (or might not) be able to prevent are more than made up for by insurance covering the loss, in the savings from an unarmed staff, and in terms of the protection of casino patrons and the guards themselves.
All that said, during and since the pandemic, violence in the tourist corridor in Las Vegas has been on the rise, both outside and inside (which used to be exceedingly uncommon, unless it was perpetrated by security itself) the casinos and we' seen more guards packing handguns these days.