The system of United States Numbered Highways (often called U.S. Routes or U.S. Highways) is an integrated system of roads and highways in the United States numbered within a nationwide grid. As these highways were coordinated among the states, they are infrequently referred to as Federal Highways, but they have always been maintained by state or local governments since their initial designation in 1926
There’s more in the wiki page about renumbering and co routing with the Interstate Highway System.
I can’t find any info as to whether there is regular funding for the states to help maintain them or the Interstates.
That might explain why transportation bills come around in Congress so frequently.