Private security aren’t invincible, and they can be overwhelmed potentially when you don’t have a degree of deterrence from regular police and the gangs can grow. Being isolated down to a compound isn’t a good idea, strategically speaking.
Also, private security may usually provide protection in your home compound, but it doesn’t travel with you when you’re out and about... shopping, sightseeing, etc. Unless, of course, you’re so rich that you have a private gang of bodyguards with you at all times. And that is somewhat limiting in terms of your interaction with the world, too.
If a big mob showed up to loot George Clooney’s house and you were his security guard, would you try to fight off the mob on George’s behalf? In a case like that, security may have a plan to evacuate the person(s) they are guarding, i.e., bug out.