Generally, they are there to, perhaps, provide a deterrent, and as witnesses to any crimes that take place. They’re not there to intervene.
A - they get paid squat. These security companies are not hiring ex cops, or even bar-bouncers, as their security. Often, they are unskilled people who lack skills to get other employment.
B - they have nearly no training in dealing with a physical altercation
If they try and intervene, they are more than likely to just get their asses beat.