A few quick points...

While this model does seem to me like it could work for some up-and-comers, especially those with local friends very invested in the writer's career, I can imagine several reasons why someone wouldn't want to pursue this course of action, even it it made sound business sense on paper.

If many fans or friends wished to help an author, the author might need to make choices about who does what, placing the author in a potentially uncomfortable managerial position over friends and other people volunteering their time. Or, said author could leave it to others to organize actions, and risk that something of less quality or speed than hoped is produced. This places the author in an awkward position: how, as the author, do you chastise, or reject the product of, volunteers or fans whose assistance you actually requested, or at least accepted? Do you endorse something sub-par to spare their feelings? How much rework can you ask them to do? Even if they do good work, how *much* can you ask of them, and for how long? How many one-act dramas will be written, and friendships or author/fan relationships soured, along the way? And how much is all this distracting you from the actual business of writing?

I have an author friend I dearly love. If there is a crisis, we always come through for each other, no matter how bad it's gotten, how much it costs or how inconvenient it is. I hope I wouldn't think twice about taking a bullet for him (though I believe that sort of thing is hard to know about yourself unless you've actually done it). But when it comes to regularly scheduled programming, I am sometimes a well-meaning flake, and distribution deadlines upon which his professional income and reputation depend are things I should not be responsible for. It's taken me a couple decades to admit this about myself; some authors' fans and friends might not know just how flaky they really are yet, and a friendship might not be worth the risk of finding out.

Now, I do think that this system you describe sounds like it could work very well for some. But I suspect that to really undertake fan-supported self-promotion, you have to risk hurting the feelings of some very well-meaning folks -- folks who really love your work and want to help, but suck at it -- for the sake of replacing them with more, and more "useful", fans and support people. I would think that some authors might just not want all the emotional turmoil that goes with that, and would rather that fans have no more expected of them than that they buy the work and share their love for it with others in their own ways. Meanwhile, people on whose active support the author is relying for his or her livelihood can be contractually obligated to perform their agreed-upon actions, while trusted friends provide emotional support and advice, help to whatever extent they wish or are able on a day-to-day basis, and serve as a highly reliable backup rescue service, just in case everything goes to hell and said author needs to sleep on someone's couch and not think about writing for a few months while he or she recovers and prepares to re-enter the fray.

So, to sum up: while I don't how Seanan specifically feels, I imagine that some authors may not want to increase the risk of personally hurting good friends, or even fans, for the sake of their writing careers, even if it means that their careers must end. And it does seem to me that adopting such a fan- and friend-supported promotion and distribution network increases that risk.