The United States is not one market and should not be one campaign.
Licensing footprints, service radii, shipping zones and state regulation all cut the map differently. Campaigns get split by state group or DMA so that bids, budgets and ad copy can differ where the economics differ. Location settings are also checked: the default option serves people merely interested in a place, which is rarely what a plumber in Ohio or a clinic in Arizona wants to pay for.