Interstate highways are owned/operated/maintained by the states. The federal government can (attempt to) influence what happens along them, but it's not going to build anything itself.
The reason we had a standardized 55 mph speed limit and 21 as the drinking age was via federal interstate highway funding being made contingent on those things.
Right, "the federal government can (attempt to) influence what happens along them." Of course, that particular brand of influence has waned in recent decades for obvious reasons.