they do that because the cover art is just for the cover. the artist can be more detailed for that than when they have to do the art for the chapters. it’s more tedious which is why they normally have different styles.
That's the whole point. When it's just a cover or just a promo, the art tends to to be more intricate, but the actual manhwa will have a much simpler style, and artists draw a cover separately for the manhwa. So switching the covers when a promo gets an adaptation would help readers go in with the right expectations.
More than once I've seen a manhwa with a really beautiful cover, only to open it and find, the cover was the novel cover, or it features the promo only art and not the entire manhwa. Though some listings here fix that by having the promo and actual mannhwa listed separately
Can they stop using a promo art as a cover???? Because it's so disappointing to find out the art is so different when you read it. No hate to the current art but.....WHY GIVE US THE PRETTY ART ONLY TO FIND IT DIFFERENT ART!? If only the current art used as a cover I've wouldn't be so disappointed