A lot of the time, if the artist hasn't reached an extremely professional level with their inking, colouring, rendering and other finishing skills, the sketch might look better to a lot of people.
Reasons the sketch can look better might be that the scribbly lines can often hide imperfections, but when you have to choose a single, clean line to ink, it'll be more clear that the anatomy or perspective is a bit off, or that the line itself is too thick, too thin or not smooth and confident looking.
Also there's the factor where adding in colour and shadow means making choices about value and saturation. Some artists might reach a pretty good level at drawing before they really get good at these, so you'll get what's a fairly decent drawing, but then the colours are muddy or garish, or there's poor light/dark balance, or maybe issues like they don't know about sub-surface scattering and shaded the skin with grey/transparent black leading to a really dead look, or they're not quite there with their understanding of volume so the shadow placement is a bit off.
Basically, because linework tends to be the first skill an artist develops, especially with comic artists and illustrators, it's usually their best skill, and it can take years for their other skills to catch up to the point where it doesn't feel like some of the energy and charm is being lost when a drawing moves from sketch to finished piece.