Many times the government can’t hire employees they need (unable to pay them a competitive salary or otherwise attractive career prospects) so can’t bring it in house.
Then, cost-plus arises because a company has to cover overhead and leave a profit, otherwise why would they bother? (Cost plus makes sense when you’re not sure enough of the requirements to write a fixed-price contract and then get change-ordered to death.)
Sure, but they may not be able or willing to commit to paying that much in the long term, or they might be unable to hire them because they're noncitizen or would fail security clearance, or they may want to hide them from FOIA for some reason, or...
Then, cost-plus arises because a company has to cover overhead and leave a profit, otherwise why would they bother? (Cost plus makes sense when you’re not sure enough of the requirements to write a fixed-price contract and then get change-ordered to death.)