Mostly because its not designed to be carried this way - in fact, if its designed as the original A1 was (like a Springfield or Colt Series 70), its much more dangerous since there is nothing between the firing pin and the primer of the live shell in the chamber (except for a spring) - drop it on the muzzle hard enough, and you risk a KB.
Plus you have to lower the hammer on a live round, which causes immediate removal of thumb, plus if the hammer inadvertently gets pulled back you're running without the thumb safety, plus you have to cock the hammer if you decide you need to use it, plus....