Well, I think there's a few things going here...
First, really good writers are not the most common things around. You have to hunt them up, and find the right venue for them - a person who is a great writer for a mundane prime-time drama may not be suited for a genre show, and vice versa.
Then comes the simple fact that the writers are not necessarily at fault for everything. Producers and network execs have a great deal of influence on what goes into a show. And sometimes they don't know what they are talking about.
Star Trek: Voyager and Enterprise, Gene Roddenberry's Andromeda, Babylon 5 and B5: Crusade, Farscape and Firefly all showed the figerprints of meddling (in various ways) by non-writers that cost the shows dearly. So, don't lay everything on the writers.