It's because the way it's made is the opposite of how digital art is actually made. things like clothing brushes, symmetry and perspective rulers, and textured brushes, just isn't used by ai obviously. because of that, human made art has a level of cohesion that ai art cannot replicate. this is especially true in shading. while ai might be able to replicate the style of a textured brush, it ends up looking like they used 5,000 different ones for no reason at all. there's no clear intention behind any of the creative decisions made. even if you think certain things in digital art are bad because the creative decision being made is "this saves time", it's still a human making a decision.

so your brain is parsing a jarbled mess, and even if you can't quite place what about it is jarbled, it's easy to pattern recognize that sorta nonsense as ai after seeing enough of it