Hey, sorry for the super late reply, was just randomly scrolling through the comments. Artists aren't just taught what a hand looks like - they're taught what a hand is. Human anatomy is a huuuuge part of live drawing. These people don't just memorize the names and shapes of your tibia and your pectoralis, they literally study how these muscles and bones deform and arrange in different poses, movements, angles, etc. There's a whole lot of deep structural understanding behind every realistic painting. That's why you usually see this six-fingered stuff of nightmares only in newbie artists/sculptors (or AI.)

I'm not even talking about the entire world of artistic knowledge that's required for a great painting - which we're nowhere close to fully understanding ourselves, let alone programming it onto an AI.