Another part of this, is its just not fun to play security. A lot of the gameplay is forced and repetitive.
As a small example
The station will always end up on red, and you will always need to convince the warden to hand out weapons that you can't be trusted with on green, because security are the mall cops. The appearance of being mall cops opens security to random acts of greytide (I am guilty of this). Random acts of greytide disarm spam leads to many security players, just being on the edge of calling it quits. Part of the reason for this appearance of being mall cops, is that most of our players are based in the United States. In the US, all police, even volunteer ones, are armed. While mall cops are not, corporate security is often armed outright, this is especially true at things like chemical plants, or research labs. The harder it is for the police to get somewhere quickly, and the more expensive/dangerous the thing in question is, the more likely security will be armed.
The private security guard who watches the self checkout machines at my local supermarket is armed. For the country most of our players live in, if you see unarmed security, their job is to call the police if they see something and not get involved themselves. (Source: I worked my way through college in Florida as a security guard.)
Security in SS13 are mall cops, they are therefore treated like mall cops, and it should not be surprising if they have the same turnover (burnout) as mall cops.