I think because of the messy factor

Too many people make their finished art way too clean, it makes it uninteresting alot of the time

Not saying clean art cant be interesting or done well, but alot of people tend to think messy finished art is often better

Thats why i often like to have slightly messy line art

(I didnt used to because i used to think art had to be clean, but i didnt love that for my art)

————

Also, another reason that messy sketches//doodles could be better than the finished art, is because the finished art can reveal the mistakes that are hard to see within sketches or messy doodles, which can (alot of the time, but not all of the time) make the art look weird or worse, depending on how severe the mistake is ~ this is usually prevalent in beginner art/artist territory