The USS United States wasn't used because a bunch of idiots in the Chair Force convinced then-SecDef Louis Johnson and JCS Charmain Omar Bradley that aircraft carriers were obsolete, and that they couldn't do anything that B-36s flying intercontinental trips from the US could.
The USS United States was cancelled shortly after the keel was laid down (1949, methinks), and the "revolt of the admirals" ensued.
Then the Air Force looked really, really f#cking stupid when
the Korean War started. The B-36 never dropped a bomb in any combat, while the Navy responded with carrier aviation to reassert once and for all that they could do the down-and-dirty...