Actually some of them do.
1st SFOD-D has been using them for a while and according to my Glock Armorer's course they were the driving force behind the creation of the extended .40 Glock magazine. I think there may be others that use them as well. But they are highly trained units that get to pick what they want, not a general issue item.
There hasn't been a Joint Service Pistol trial for Glock or any of the new polymer handguns to compete in.
The one that chose the Beretta happened before most of the polymer 9's existed.