Well, first off the pool is lower because you can’t just grab some rando off the street. The whole job is protecting the hen house and you absolutely can NOT put a fox in that position. Reputation, etc:

You also can’t put someone who can be turned into a fox into the position. If you’re protecting something that could potentially be worth billions stolen, you don’t want to pay starvation wages where the instability would make the risk seem lower.

If the person you trust now also has to move, then you also have to pay extra for that.

Every movie makes the security guard a big guy falling asleep in front of an array of monitors but that’s not the case if the thing you’re protecting is worth a LOT.