While we're at it, might as well ask why artists don't just make more money if they're so starving. It can't be that hard. Besides, how much can a banana cost? $10? PULL YOURSELF UP BY YOUR BOOTSTRAPS!!! /s
Anyways, there's several reasons why an artist may not improve and most of them were already touched on here.
The artist could come from a disadvantaged background where studying as a skill just wasn't taught, so studying and learning may be more challenging (and if you come from a poor background you know exactly what I mean)
They could be disabled, whether that be a learning disability, or some sort of physical impairment that limits their artistic abilities or ability to improve as quickly as an abled person.
Perhaps not even disabled, they could even have some sort of disorder or be neurodivergent.
Some people just plain aren't good at studying and learning things. We can't all improve at the same pace. Some people may even be the type to regress before improving.
Or maybe they just don't want to improve. Or they don't have the time to improve. Becoming the best doesn't have to be every artist's goal.
You never know what kind of circumstances somebody has.
Edit: I was working on this post on-and-off, ended up being late to the discussion lol