Hollywood has to appeal to the broadest audience, and when it comes to most social and economic issues, America is progressive. Because of that, the messages that are in Hollywood movies tend to be, for instance, pro-environment.
Everyone is sort of in their own little area counting lines and no one talks when film's not rolling. There's constantly actors coming to me back behind the monitor screaming at me, "Why did my line count drop?" It's a nasty tense environment.
The only way that Hollywood ever skews toward liberal is because part of what we make out of Hollywood involves writers, actors, directors, musicians, set designers, and photographers. In general, people like that are going to be more progressive, more open minded, a little more altruistic.