But for the “main” art—there’s the penciller & inker. These are usually two separate people. Following those are the letterer (me) & the colorist.

To produce a monthly comic book of 22 pages of comics, we generally need to create one page per day each—IF we want weekends off.

As a long-time letterer who was an early user of computers in comics creation, I can do a whole issue of lettering in about 4 or 5 hours. This morning I did 18 pages before lunch. Obviously lettering is much less time-consuming than drawing each panel, though. And I’m very fast.

But even that time-frame is aided by a particular workflow that I’ve designed over time to speed up the process. Other letterers may take 2 days to do a whole 22-page issue.

If you think about the process on a particular Tuesday, the penciller may be pencilling page 7 of Super-Guy #321. That same day the inker is inking page 6—that he got from the penciller the night before. The same day the colorist may be coloring page 5—that the inker had sent the colorist the night before.

The production chain isn’t really that tight—a penciller may deliver the art to the inker in 5-page batches—but they are staggered by only a week or so in a well-run schedule.

So once a penciller is done, within a week or so everyone else has completed their job.

By the time the issue is in the comics shop—the next issue should be about half-done or completely done already…and in the shop complete in another month.

The penciller creates a drawing, the inker outlines, interprets, finalizes, retraces this drawing by using a pencil, pen or a brush. Inking was necessary in the traditional printing process as presses could not reproduce pencilled drawings. "Inking" of text is usually handled by another specialist, the letterer, the application of colors by the colorist.

As the last hand in the production chain before the colorist, the inker has the final word on the look of the page, and can help control a story's mood, pace, and readability.

A good inker can salvage shaky pencils, while a bad one can obliterate great craftsmanship and/or muddy good storytelling

Because even without taking into account the fact that it was drawn to allow the colorist more latitude, it seems pretty detailed to me. And Porter’s background buildings here don’t look CG at all, just, well, detailed.

I think what you’re noticing is that artists often don’t use line shading these days as much. When I first began collecting comics in the latter half of the silver age, inkers were also called embellishers. Right around the time Jack Kirby was making the New Gods, the first hint that this would fall out of style took place.

Namely Kirby stopped working with Vince Colletta over changes he would make to the artwork. Colletta, one of the fastest inkers in the business, had a habit of erasing elements he felt were superfluous. Parts of architecture, people, whatever simplified the process. He did it a lot. Colletta took liberties, and became an example of what not to do if you’re an inker.

What inkers were doing began to be looked at more closely, and eventually embellishment was discouraged altogether. Now inkers are expected to simply ink what’s there and nothing more. Which is sad because it means comics have less character.

However, the trade off is that today coloring is far more sophisticated and embellishment is the colorist’s job. It can look what some call Photo-shop-y, but when done well looks fantastic.

And because of improvements in resolution, inking is actually more detailed these days, not less. Those fine lines detailing the architecture in the background would have been invisible using the old process.

1) Price.
The Japanese manga magazines are phonebook-sized (Harry Potter-book sized for the younger-set who've never seen a phonebook) weekly magazines that do their best to allow even elementary-school kids to buy it without breaking their allowance. So the magazines use very cheap, recycled paper and only one color of ink. So you get some 300-600 pages of manga for under $5.00. And although they have some advertisements, it's relatively few. Again for the most popular magazines, they come out every week.

US comics tend to run about 22 pages of artwork with quite a few advertisements every month. And those run $2-$5.

2) Distribution of work
Manga are essentially done by one person. That means for most manga, you have to draw and ink 30-40 pages of manga in a month* all by yourself. Any colored pages are also done by the artist alone. The artist also does sound effects themselves. (The artists working on weekly manga may have assistants, but they are mainly for grunt work of filling in black areas, background details, and laying down tone. It's the artist who usually still draws and inks the characters and also does the coloring for the rare colored pages.**)

In comics, there is usually a separate writer, penciler, inker, letter (who not only does in-balloon lettering but also sound effects), and colorist. If the work of the other artists is done far enough ahead of time, the colorist has plenty of time to do his/her work.

3) Time before deadline
The system for US comics starts the process for each comic months in advance, giving each person in the process enough time (theoretically) to complete their work.

Manga artists tend to work right up to the printing deadline. There are many stories of editors waiting on the artist's couch, then rushing the finished pages directly to the printer.

4) It's art
Have you ever seen a really well done black-and-white movie? If not, try it sometime. They convey a certain mood, especially in the use of stark shadows, much better than color can. Manga, as an industry and artform, have built up plenty of excellent techniques for how to use black-and-white art, and in some cases (as exemplified by Marvel's and Viz's early experiments with colorizing manga in the late '80s and early '90s), the color actually lessens the impact of the artwork.

There are probably other reasons that I'm forgetting, but I think those are the most major reasons.

*Although the most popular manga are weekly, most manga artists in the industry work for monthly magazines, and don't make enough to afford regular assistants.

** There are reports of certain very popular manga artists who basically have their assistants do all their work for them. I'm sure there are examples out there, but as far as I can tell, this is not true for most of the manga artists.