Good question. A little disclaimer first: Most of the people that work for Blackwater are honorable and patriotic just like a Soldier on the US Payroll and join Blackwater and other PMC for various reasons ie retirement, injury and some are in it for the money.

With that said you have to look at how many Soldiers do stay with their perspective units IE Ranger, SF the reason is that they are in it for their country and money is not the issue. It may be hard to fathom but people put themselves in harms way for their country not the thrill of combat or getting shot at if money or thrills was the case the entire military would be privatized.

In short: Esprit de Corp