It is because the Glock “safety” is embedded in the trigger mechanism itself. That means when holstering, if anything gets in the trigger guard, you can end up pulling the trigger. By anything, I mean not just your finger. It could be a fold of clothing or a drawstring from a wind-breaker. Hence if you don’t pay attention while holstering your Glock, you can get “Glock leg.
It is possible to do this with other pistols, but Glocks are ubiquitous now-a-days. It is possible, but more difficult to do this with a revolver due to a stronger (10-12 lbs) and longer trigger pull.
C.W.
Interesting note about the Serpa retention holsters. My current holster is good old plain leather. However, I'm looking for retention holsters for both my G19 and a full size 1911. At least for the G19 I think I'll favor the ones with the thumb release vs the ones with the button/tab by the trigger guard...