Considering the astoundingly high incidence of Glock owners shooting themselves due to bad re-holstering techniques (as compared to other types of firearms) I would speculate that Glock doesn't want to take the change on getting sued when someone shoots themself in the foot using one of their guns AND one of their purpose-built holsters.
I'm not saying that Glock owners in general are any less competent than any other type of owner, but I think it's pretty much an established fact that Glocks are involved in more "negligent discharges" than any other kind of handgun.
Just ask Washington DC's Metro Police. When they switched to Glocks from revolvers in the late 1980s:
In the first 10 years DCMP carried Glocks, they racked up 120 ND's, killed one person they didn't intend to kill, wounded 9 innocent citizens. and nineteen officers shot themselves or other officers when they didn't intend to.
(an interesting side note: if you look at the map of NDs, 17 of them occurred OUTSIDE of the District--in MD. OUTSIDE of their Jurisdiction. And most of them were WAY outside--not like they were chasing someone on foot near the border, I mean like over 5 miles into PG county...WTF?...)
Sure, I think we can all agree that DCMP is perhaps one of the most under-trained, incompetent, and goofball-ridden metropolitan police forces in the nation, but that's a LOT of bullets flying around that should have never left the gun.
The Glock is a fine weapon. In trained hands it is inherently safe, and VERY effective and reliable. But there seem to be a LOT of folks carrying them--professionally and as "civilians"--that just don't know when to keep their fingers off the bang switch.
I imagine Glock knows that, and just doesn't want to risk any more litigation than they already are...
Maybe you can find one on EBAY...