I'd like to go slow here, as I THINK I understand what you're saying. If so, my first response might be "are single actions the only handguns you ever have, and ever will fire?"
Why ask that?
1. Trying to combine the above concepts and answers, the clear answer is because of muscle memory. Every motor coordination activity is perfected by muscle memory, the unconscious movement repeated so often as to become ... well, unconscious. Introduce any variation and that memory may prove faulty and result in a mistake.
2. If the answer to the posed question is "yes" the next muscle memory issue becomes "when?" The draw, the trigger finger engages, and the hammer is cocked ... I'm sorry, when? Between holster and horizontal? Sometime when the muzzle is pointing upward on the way to vertical? Somewhere the opposing thumb cocks, and in each case where is that "somewhere?"
That's a lot of blada blada. Sorry. I teach my grandkids the same thing (muscle memory) the same way, every time regardless of make, model, action, long gun, short gun, etc. The muscles take a long time to remember, and just as long (or longer) to unlearn.
-jb, more comfortable as a simpleton