It's mostly bias in the training data. Most people aren't posting mediocre images of themselves online so models rarely see that. Most are also finetuned to specifically avoid outputting that kind of stuff because people don't want it.

Out of focus is easy for most base models but getting an average looking person is harder.

you can do that with stable diffusion and loras, yes

On the focus part, I've seen some impressive results from people who input a specific camera, lens, and focal distance.

The models are deliberately engineered to create "good" images, just like cameras get autofocus, anti-shake and stuff. There are many tools that will auto-prettify people, not so many for the reverse.

There are enough imperfect images around for the model to know what that looks like.

I assumed this was because it's making an average. Human attraction is highly sensitive to symmetry so this creates that symmerty by the way it works.