U.S. don't want to look weak and make concessions to Russia. If that persists, it will continue to increase the probability that Putin will act and invade, to prevent Ukraine in NATO once and for all, for moderate/low price of sanctions.
Everybody including Putin knows that NATO is divided on this and won't put its full weight behind Ukraine. When Putin is taking control of Ukraine, U.S. will boost its presence in Eastern Europe substantially but then may want to (and have to) talk to Russians again to establish some predictable environment.
If U.S. and war-hungry European states keep their insane stance, behaving as if Ukraine is somehow already in NATO, the result will be lots of suffering in Eastern Europe. It would be best for U.S and Europe to change strategy and let Russia and Ukraine deal with their problems themselves and maybe help in negotiations. Like Germany is trying to do.
