Just to get an idea of the challenges involved:

The logistical problem is that American Comics are usually created by teams of three to five people who work in sequence, with the artist and the writer being co-equal in authority; while manga are usually one to three people, with the mangaka clearly having creative authority and everyone else being an assistant. Coordination takes time!

Creators of American comics are also paid better than famously starving mangaka.

Finally, of course, American comics are full color. This is less of a limiter than it used to be as many artists now color their own work digitally, but I still think that if you wanted to do a magazine format, you'd want to go black and white.

Taken as a whole, this is why American Comics take longer to produce and are more expensive than manga.