> If anything we'd get more economies of scale.

No, because there are also diseconomies of scale. Some services are best provided by smaller-scale organizations than an entire nation, much less one the size of the US. It is perfectly plausible that a service that worked well for the UK might not work at the scale of the US. Making an organization bigger means adding more layers of bureaucracy, which puts the ultimate decision makers farther away from the problems they are trying to solve. It also creates more room for resources to be absorbed by political infighting within the organization.