Typically you get a few good engineers and then eventually they get swapped out mid period of performance for "other projects" (or leave their firm) and there's not really a good option for saying "no" without killing the whole project and going through proposal overhead again to encounter the same problem.

The ones that do stay in that environment either have golden handcuffs or are goddamned patriots and love the challenge. I like to think many are the latter as government is an amazing place to do impactful work that is too important to screw up.