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To use my gun, I need to draw it from the holster, keep my finger off the trigger, take up my sight picture, put my finger on the trigger, and pull the trigger. If it has a safety, I also need to ensure that my method of drawing the gun and taking a grip deactivates that safety. I want to make sure that I can do all of these things naturally, and without thinking, while I am in a high stress situation. So I practice. Ad nauseum. I am confident that when I take my grip during the draw, my thumb will come down and apply pressure to the top of the safety before coming to rest on top of the safety lever. The reason I am confident is because that is exactly where it goes every single time I draw my gun. And every single time I take a firing grip at the range even if I am not practicing my draw, even if I am using a gun that does not have a thumb safety.

I would no sooner "forget" to deactivate the safety than I would "forget" to take the gun out of the holster, or to acquire a sight picture, or even to pull the trigger. There is actually a greater chance of forgetting to do any of those other things because they are things that I actively do as opposed to my thumb position which is a natural result of gripping the gun in my hand.