Until the 1990s (at the latest) the U.S. had a policy of widely distributing government agency offices throughout the country. This 1) kept power (agencies, lobbyists, workers) from centralizing in D.C., and 2) spread government expenditures. There were other reasons for doing this, especially before the telecommunication era. But for the middle part of the 20th century this pattern held strong even as transportation and telecommunication exploded.

But there's always the risk of a pathological co-dependency on government employment. You can see this in many developing countries. So for partly legitimate reasons, the Civil Service has long been the boogeyman of Republican politics. Specifically, that civil servants are reliable "big government", Democratic voters. Better to 1) pack their voting power and 2) prefer private out-sourcing. This is one major reason (if not the major reason) why government agency activity and policy have migrated to D.C., and why the headquarters of so many corporations have moved to the D.C. metro area.