There's the trade-off. If D&D was my only hobby, I might be willing to sacrifice more hours to it. But it isn't. I've got my girlfriend, I've got reading (non-gaming materials!), cycling, hiking, backpacking, photography, a well paying job that I intend to keep (which can mean long hours), non-gaming friends, my family, etc. All of these things take time, so aside from weekly write-ups for the game, vetting the PCs, and the occasional conversion, I can't spare more time for D&D.
And most of my players are also busy professionals. We get together every week, get in 3 hours of gaming and 1 hour of socializing, and that's it. For us, this is a hobby, not a lifestyle. It's a game, not "immersive roleplay", so yes, it's too much work if I have to do much more than read the adventure/campaign setting. I love Monte Cook's work, so if he decided to do a "Shackled City" or other type of adventure path, I'd buy it even if it was $200.