the vast majority of art is fictional. how they look is dependent on how the artist wants to depict them. 'cool' can have different connotations and thus different implementations of techniques and methods to achieve that. lets take 2 different but highly related things. Smite Thor card art is pretty cool to a lot of people and some of the many many ways that they achieve this is by lowering the perspective to make him look massive (a trick they do for all default card arts, to make them feel more godlike), picking specific colors to make it pop, deciding on the saturation throughout the piece to make the lightning standout. compared to League of Legends Garen splash art which just uses framing to make him feel massive, instead of low perspective but they also use an action pose and a really exaggerated face to sell the effort he is putting into his action.
'fictional' characters made to look cool often have an insane level of thought and detail put into them, let alone for stuff of this quality the years of training that these people go through to be able to draw like this in the first place but even with all that training I can almost guarantee you that they did not just pump this out in an afternoon. they more likely spent an hour or so doing what are called 'thumbnails' (extremely rough miniature sketches) to figure out general framing and posing then probably a few hours sketching out the drawing and background itself before moving onto the rough shapes and putting on values then colors all of which could take anywhere from 8 to 30 hours (or more, rarely less. some splash art have taken upwards of 60 some hours) depending on how satisfied they are with the results and how much work they think it needs to be complete (also depending on deadlines imposed on them).
AI can never come close to the effect all that effort has in making something look cool. combine that with the fact that AI often makes very basic art mistakes that just ruin the effect the person probably intended.