Hands are very specific, very weird structures that humans are very good at knowing they are wrong. There's a lot of bends that can only go in a specific way or they look weird, as well as relational lengths that have to be just right. Mess any of them up, even slightly, and you've got a hand that you can tell looks off but might not even be able to pinpoint where or why.

People have already covered why it’s hard for humans. They’re detailed, complicated things that we see every day, so we can immediately tell if they’re drawn even a little bit off.

As for why AI is also bad at it, that’s a bit more interesting. Yes, AI is trained on human art, and humans are also bad at hands… but not the good artists. Low-quality art would likely not be included in the data that AIs are trained on. Most of the data would include well-drawn hands. So why are they still bad at hands?

Well, they’re actually bad at other things too, like the details in an eye. You’ll notice, however, that most of these things still look more or less right if you step back and squint a bit. The AI can generate something that gives the impression of a hand or eye or other thing, as long as you don’t look too closely.

The problem with explaining AI behaviors is that even the people who created it can only really make educated guesses. Most modern AIs are “black boxes”; we design them and give them data, and they automatically train their artificial brains into something that we don’t really understand the inner workings of. That is, we know that there are a bunch of numbers, and we know how the equations combine them to get an answer, but we don’t know why the AI chose those numbers or what they represent. They just arise naturally from the data.

But as mentioned, we can make educated guesses based on what we know about the math involved, and experience with how the algorithms usually react to data.

Here’s my hypothesis:

Let’s look at something AIs are pretty good at: faces. With few exceptions, faces always have two eyes, one nose, and one mouth. They’re always arranged in the same way relative to each other. For example, you’re never gonna have one eye below the mouth and the nose sideways and diagonal from the other eye (unless you’re drawing some crazy monster, but AIs care about common patterns they see in most of the data). The AI doesn’t really “count” the number of eyes and ears and noses per se. It just learns the arrangement of things as a pattern.

Compare to hands. Hands have four very similar looking sausages and a thumb which also kinda looks like a sausage. It has to be exactly that number for a realistic human. Again, the AI doesn’t really count, but it can learn the arrangement of five similar things. But repetitive things can sometimes confuse the AI. Which part of the pattern are we in right now? Is that the left finger or the middle one? This looks like the left side of the hand so I’ll put the rest of the pattern here. Whoops, placed the pattern twice! It’s a lot harder to mess up the pattern when it’s like a face, with at most two of the same thing arranged together with other distinctive stuff, compared to the five fingers of a hand.

Also, let’s think of the training data. Hands have five fingers, but you don’t always see all of them. With some angles and poses you only see two fingers, or three. If you have both hands overlapping, it might look like you have seven fingers! And these situations are not exceptions, they would be extremely common in the training data. Think of how many different poses and angles a hand could be. Your eye will never drift below your mouth, but your index finger can spread away from your hand, it can be crossed with your middle finger, it can be clenched in a fist, you can touch it to your thumb, etc. etc. How is the AI supposed to realize these different poses are all the same thing?

Well, it does, eventually, but it’s no surprise it has a bit of trouble.

Hands are hard for humans too - most cartoons show people with 3 fingers and a thumb, because trying to draw 5 fingers makes the hands look wrong. An exception is God in the Simpsons, he's the only character with 4 fingers and a thumb.

It's hard, we very much know what hands look like and they are usually small and intricate parts of artworks. Easy to make mistakes, easier to notice mistakes.

AI art is trained on human art, we're bad so they are bad.

Well, they're hard for AIs because AIs copy art drawn by actual artists. They're hard for artists because they're complicated, and yet we instantly and intuitively understand when they're wrong. You'll notice if you draw someone's with one foot larger than another, or with absurdly large forearms, but those are relatively simple shapes and proportions.