I literally love you.
Personally, I put effort into my covers because I have the brainworms and feel anxious if I don't feel like I've put in my best effort. Even if I do not have the time/energy, I usually put in a really basic text placeholder that nonetheless looks nice.
As a reader, I judge books by their cover. I won't say that a good cover is a decider for me, but it definitely draws me in initially at all. I browse SH like I browse a bookstore: the pretty ones catch my eye, and then I read the blurb and decide from there.
As others have said, it indicates pride and, I think confidence from the authors. I respect that not everyone is an artist, or has the money or friends to get work done, so I don't just judge based off pure artwork. I think there's some really nice basic graphic design covers on this website.
I tend to pass on stories that still have the default SH "cover", not because I think the author doesn't care or they aren't as good, but because it usually indicates a story is still new and doesn't have many chapters. I like to start reading when stories have around 8-12 chapters or so (depending on wordcount), and if the author doesn't even have a cover yet, they probably are still in the early stages of the story and haven't considered making a cover a worthwhile time investment yet.