I know I shouldn't be cribbing from James Carville, but I think that description is pretty apt for why, despite the fact that all of the networks have made a concerted effort to put content on the web for free, people still keep turning to YouTube.
I realize that my experience may be tainted by my status as a Mac user (we represent only 3% of the market, why accommodate us?), but when I tried to watch television episodes, or even previews, on NBC.com, the site would load the advertisement that plays prior to the video correctly, but couldn't seem to make it all the way through the actual video without freezing. After trying to reload the video three times, I turned to YouTube. Sure enough, there was the video I was looking for (several copies), and with one click it started loading. Since YouTube does not insist on streaming the video, my lower-speed wifi DSL was able to handle loading the video, and I could watch it without the desire to throw my mouse at the wall.
I think that this frustration with the tempermental nature of is all part and parcel of what should be a cardinal rule of technology - the more buttons or hoops you add, the less people will want to adopt your new technology. Outside of the type of people that read Engadget, there are plenty of people who navigate the technology of modern living like water in a stream - they take the path of least resistance. Since YouTube does not require you to a) install plugins, b) register, c) watch ads, or d) visit several sites to get the content you want, it is hardly surprising that people take that option. I think the networks would do well to remember the old adage - "once bitten, twice shy" - a bad experience the first time someone visits a site, the less likely it will be that they'll try again tomorrow.