[RugnirViking]

But aren't they explicitly paying the cost of those employees' competitive salaries?

[lmm]

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...

[User23]

Or campaign contributors expect a return on their "investment."

[sokoloff]

Yes, but not via the GS pay schedule: https://www.federalpay.org/gs/calculator