"Why doesn't the US military use Glocks?"(OP).
Perhaps our Military leaders don't drink kool-aid.
Seriously though, RICKB had it right... "Because the U.S. government has no interest in basing missiles in Austria?"
Politics had more to do with the M-9 being chosen than most realize. The Beretta is a good gun, so was / is the Sig, Ruger & S&W that were considered. Politics.
If common sense was involved, Colt would have been contracted to produce new .45 ACP 1911A1's.....