Possibly because state law requires it?
I am aware the state law requirement. But that does not answer the question. Why would a security guard need a PI license? Most state PI license requirements are there to address the reliability of rendering evidence or expert testimony in court. For example, if you are a cyber security investigator and might be called to court to testify on a security breach, then in many states, you are required to have a PI license.
From that perspective, there is little to no need for a PI license for a security guard. If the purpose is to track bouncers or other security officers, then I would think that a different set of “controls” would be appropriate. I would speculate that “bouncer/security” laws would sheild the individual from certain civil penalties or perhaps even grant quasi-police powers. But I would expect such controls to include background checks, training on apprehension, police procedures, etc. Does Nevada require that under their PI license? If they do, that would seem overkill for the actual private investigator (cyber security example from above).
Now if the purpose is to restrict employment, prop up salaries, etc Ok, perhaps that is a reasonable position.
I guess the better phrasing of the question would be: What state interest is served by requiring a security guard to have a PI license?
Just does not make sense to me - maybe someone can explain it.