Safeties are really great mechanisms to prevent the excessive reduction of a companies finances due unexpected lawsuit. And that's about it.
Even the NRA warns that safeties are mechanical devices that can and do fail. They are no substitute for safe handling.
Disclaimer on my part, I'm not saying "screw safeties!" I'm just saying that putting all your faith in a safety is likely to cause an accident either because of mechanical failure or user error in trusting that the safety is engaged while you accidentally shoot your buddy. Using a safety but not trusting it is a more prudent approach. So, you know, don't go walking around pulling the trigger on a cocked and locked 1911 to prove it's on safe. By all means, use the safeties in the way they are intended.
Now, pure personal opinion. If Glock added an external safety it would never be engaged if I owned it. That's personal preference that I know not every one agrees with. For me, I shoot mainly striker fired so I don't practice functional operation of a safety. Putting one on would inevitably lead to an engaged safety when I don't want one.
Fact is, with most striker fired pistols, there is no way to fire the gun without pulling the trigger. There is no spring set under tension or compression that is just waiting to be loosed so that the striker can fire. In order to put tension on the striker spring, and therefore energize it, the trigger bar has to be moved. If it's not, there is no tension on the spring. The striker block can't be moved out of the way unless the trigger bar is moved. Etc... It's just not possible to fire them without pulling the trigger back. Now sure there are ways to pull the trigger unintentionally, like maybe getting something wedge into the trigger housing. But the trigger has to be pulled.
In hammer fired pistols, a spring is under tension or compression if the hammer is cocked. It's not at steady state. So the failure of the sear connection to the trigger could result in a released hammer without pulling the trigger. I know it's not likely, but it's physically possible because the hammer spring has stored energy as opposed to a de-energized steady state. So external safeties on hammer fired guns are there to prevent the hammer from being able to contact the firing pin in the even that it accidentally let's go. Grip safeties, firing pin blocks, etc are all there to stop that from happening. Which is why a 1911 is intended to be carried cocked and locked. The thumb safety isn't the only safety on them. There are "internal" safeties as well. That way, if for some reason the thumb safety fails, the grip safety and the pin block prevent the gun from firing.