The Glock is safe to carry without an external thumb safety because the Glock pistol is not carried in a cocked condition. Same thing with a revolver or a DA/SA semi auto with a decocker. Same thing with the DAO designs.
The external, mechanical thumb safety is a design feature that was originally invented for use with single action, semiautomatic handguns that were intended to be carried “cocked and locked”. John Moses Brownings designs; the 1911, the Hi-Power, et al. are some of the most famous examples. The CZ 75 is another example. There have been single action, striker fired, semi autos which use a similar firing actuator as the Glock which have been intended to be carried “cocked and locked”, the P-08 Luger is probably the most famous example; it has an external, mechanical thumb safety.
But these are all single action designs and it was for this action design that the external, mechanical thumb safety was originally designed. Over the years, there have been mechanical thumb safeties installed on non-single action designs. Glock has even come up with a factory designed thumb safety (requested by the Tasmanian Police, IIRC). But these safety additions are redundant on non-single action design pistols.
And BTW, the same basically holds true for long arms. The AR15 is a single action, semiautomatic rifle and guess what? Yup, it’s got an external, mechanical thumb safety, too. Why? Because it's intended to be carried cocked!
So, this whole thing is really very simple. If a weapon is intended to be carried with the action in a cocked condition, then there needs to be a separate mechanism to "lock" the action. If it isn't carried cocked, then there isn't any real need for that design feature.