It's not that. It's that once you know the best move you start to see why. And then it becomes hard to get it out of your mind. It's the reason I don't recommend computer assisted game analysis, unless you just want a quick blunder check.
A very non-zero chunk of people also probably would not have even understood that that's cheating if it wasn't really clearly laid out in the interface somewhere. For instance computer assistance in the largest correspondence chess league is legal.