It's because the media, who teach most people how to think, is mostly owned by corporations who benefit from pro-corpo status quo policies.

There's also the fact that this, "centrist liberal," strategy worked exactly once in 1992 (and that may have had more to do with Ross Perot than anything else), but now there's an entire pundit and strategist class built around it. Most of these people don't mind losing elections if it means they can keep their jobs.