Rand Simberg has a theory. An excerpt:

Here’s my theory. Even ignoring the fact that a lot of Hollywood writers tend to be leftist, some of them may actually have personal reasons to hate “big business” and think it venal. For them, it often is. First of all–they work in Hollywood, for those paragons of probity and above-board accounting, television and film studios, and production companies. And horror stories about them abound. One could easily see why, if that was the only experience one had with the business world, that one would have a pretty jaundiced view. But there’s another part that is less obvious. People tend (rightly) to write what they know. And when screen writers are between screen-writing gigs, who do they work for? Well, here’s a clue. What is one of the most common businesses to be depicted in television and movies? Think, for example, “Bewitched.” Or “Thirty Something.” That’s right. Ad agencies. I haven’t done the research (it would be a good thesis project), but I’ll bet that television and film characters work at ad agencies vastly out of proportion to the number of people who do so in the real world.