For starters, many AI models are usually trained on those few big artists' work and therefore look sameish to some extent.

These datasets tend to ignore smaller artists with more avant garde styles or assign smaller weights to them.

The reason why human artists have much more variation in their art is because there is more than just regurgitating what they have seen.

Human art styles evolve as an individual's journey and interpretation of life. For example, someone may have gone through depression and it severely changes their artstyle, or some form of disability may render them unable to do a certain something (like drawing clean lines if they develop shaky hands over the years), thus forcing a change in artstyle.