In other government/megacorp settings, contractors can end up staying in the same position for years working for different companies. Back in the early 2000's I worked for a State government where some of the contractors had been around for twenty years working for Digital, IBM, Dell, HP, a half dozen companies you never heard of, etc.
There tends to be a small world around big organizations, as social proof is important for getting jobs and closing deals.
I'll just throw in my two cents:
It seems pretty simple from my point of view. People switch between companies that win bids and contracts. Those companies now need to staff and they staff with the people from the companies that lost bids. The people that are picked up happen to know the systems that they're dealing with.
Also word gets around to which contractors look "hot" compared to others. They start getting resumes from people looking to jump ship.
Combine that with laws that stipulate some amount of contracting money must go to small minority owned businesses... yeah.
There tends to be a small world around big organizations, as social proof is important for getting jobs and closing deals.