The budgets are really different but it’s because the cover is often a novel cover which has a very different budget. The artists draw the best that they can, it’s just that the best artists are insanely expensive. You usually see the best art in adaptations of the most popular novels. Becoming the Villain's Family got the cover artist as the artist for the adaptation, you can see how much better it looks. Shards of a broken glass slipper likely is the same

I guess all the budget goes to beautiful cover art and the chapter art is so piece of shit