For me it’s because, at least historically speaking, there’s usually no immediate feedback whatsoever if the command was successful or not. And then in addition, individual applications can also disable it entirely, often with no indication. Then on top of that, I switch back and forth between OSes, and some like Mac have different command keys and my brain doesn’t always automatically do the translation – especially since I’m using the same keyboard for all of them.

So, from a strictly reactive sensory perspective, my failure rate with just the single ctrl + c command is far from the non-zero it would take for my brain to be convinced that just the one ‘c’ is sufficient. Hitting the ‘c’ many times also gives my brain a tiny bump of endorphins / dopamine / whatever over and above hitting it just the once. It’s like playing the lotto or playing a slot machine, except all I win is paste.