For me at least, it really is the art. The proportions are absolute goals. As an artist/graphic designer/I-draw-things person, I am absolutely green with envy over the author's anatomy, command of lighting and staging, and even creature design to some extent. It's rare to see such svelte curves to bodies in anime art, because usually they're significantly more exaggerated.
There's probably more I could rant about with the art, but the fact remains that I'm absolutely charmed by it.
As far as story goes, I do know it's been "done before" to some extent, but just because it's been done doesn't mean it's been done well, or in a way that I appreciate in particular. That's not to say that Solo Leveling is the epitome of its kind — it probably isn't — but it is an extremely solid and satisfying offering if nothing else.
Overall, I rate the story an 8/10 and the art a 10/10. It's not perfect, but it's just pretty damn awesome by my personal standards. ovo
Your opinions may differ, but you just have to understand that you must be looking for something else that Solo Leveling isn't offering you. When everyone around me raved about KonoSuba, I fell in love with Hoozuki no Reitetsu instead, because my sense of humor is different, as an example. So while I can't tell you why you in particular do not enjoy Solo Leveling, I can tell you why I liked it. <3