Doesn't the rest of the stuff in landfills stay there forever, too?
But regarding Glocks, I suppose in some ways, as far as any military is concerned, it's just another pistol and as such, way down on the list of important things to worry about. Pistols are generally not issued very widely in most armies, although some units may have a higher proportion. Tankers usually have a pistol but even that seems to be changing. My son was a tank crewman for three years, of which about 15 months in Iraq. His first photo sent home showed him with his pistol in a drop leg holster and a shotgun, of all things. I think he told me that was just for the photo (which suggests all photographic evidence is suspect). Anyway, they later turned in their pistols. Everyone carried either a carbine or rifle. All the other photos had tank guns.
One could just as easily ask why doesn't the army used Brand X pistol. After all, not every other army uses Glocks. If you like Sigs or Berettas or S&W, a Glock is just another pistol. Beretta just got in first.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.