I'll tell you why!
...Rather, it's more accurate to say...I'll SHOW you why.
And I'm not even done yet! This image could pass for finished, yes? Well, it's not! You might not be able to tell thanks to the overlaid original-sketch hiding it well, but a lot of the detail work has yet to be done. The socks need to have their texture done to look more like actual socks. (Right now, the stuff you see there is the original sketch at low opacity, which is a neat effect, but not good enough.) The hair similarly lacks the linework which makes it more hair-like. (All the work you see done already is, again, from the original sketch.)
Furthermore, the secondary colors for her skin, hair, and hoodie haven't been added in, when they are absolutely vital to give her a more lifelike look. (Right now, the coloring's just the flats with some detail-work.) That doesn't even begin to cover that her hoodie also needs texture-work done, so it looks more like clothing! (You can see quite a bit of this in the sketch overlay, but again, the sketch isn't good enough, here.)
This doesn't even go into the highlights and shadows I've yet to add. I have added in a backlight (a yellow light that was copied from the lineart, put behind it, and filled in to be a solid silhouette covering everything that's, well, colored, at some reduced opacity, then turning the lineart off--this, combined with the flats not being at 100% opacity gives a 'shine' from behind) and a front-shadow (a copy of the backlight layer, colored black, put in front of everything except the faded sketch, and reduced WAY down in opacity), but these are cheap tricks.
They happen to be cheap tricks which create a super-cool effect (this image would be 10% less badass without them), one of two that I employ (the other being the sketch overlay, which makes the image be at least 30% more badass), but they are not proper lighting/shading, just shortcuts to them.
When I actually do lighting/shading, I usually use an airbrush with solid white (highlights) and solid black on two separate layers, and toy with the opacity until it looks about right for what I'm going for. (Said highlights/lowlights are just me improvising for the most part off of instinct on where I "feel" the lighting would go. I am fairly incompetent at making these predictions, but I do at least try.)
So I'd say I'm about half-finished, in spite of around 5-10 hours of work on the image thusfar.
Remember, this was my earlier art, which--while more detailed in some ways--was mostly sloppier than my later stuff. So this is the quality of what I'll be doing at my lower points. And it already looks this badass. In spite of not being done yet.
I am going to seriously enjoy making this comic, given that this is what I'm making right now.
It's just that...well...it's incredibly time-consuming! I know I'm a good artist. This type of stuff is not something which is easy to sculpt. It's a delicate craft, one which takes a lot of time to master. And yet, I am building, I am creating, it all the same. I'm just...not exactly the fastest of artists.