Because it takes a small amount of space to convey the idea. I think we can agree on at least that much--a cover should convey something about the book. How we envision that outcome is probably something we can't agree on.
I can tell you what doesn't work for me--bad vector art, amateurish artwork, poor photography, images pasted together that weren't meant to go together (size differential). Anything sims I think. I've never seen it work. Covers that are so dark I can't tell what's going on, covers that have too much going on.
Here's a site where you can look at hundreds of book covers and see if they work. If yes, why, if not, why not.
Book Cover Archive. There are links to the sites of individual book designers as well.
Maybe you're just not interested in designing a cover. I understand. 4 years ago I didn't aspire to editing, proofreading and formatting. I didn't dream I'd be making covers. But publishing isn't the same as it was 4 years ago. You have no publisher to do all the work for you so either you do it or you pay to have it done. I'm a DIY type. We all need to be sharper about how we present our work and covers are part of it whether you do the work or pay someone to do it for you.
Here's a portrait of some famous men in their youth. I suspect if you look at it long enough you will figure it out.