Because most of these people make cheap potshots, even the ones with evidence to back themselves up. Just look at John Stewart, he can be incredibly insightful... but the majority of his comments during interviews are sarcastic cracks about how things aren't working as they should be.

The basis of a lot of left-leaning philosophies are abstract and complicated. That makes it hard to sum them up in convenient sound bites that are easy for mass audiences to consume. It also means that it can be very fractious, as the complicated nature of these philosophies lead to a wide variety of nearly-the-same-but-not-quite conclusions. Considering the very black and white, good versus bad, mentality that has been most prevalent at least since the WWII (if not earlier), this generally leads to people being unwilling to work with people whom they agree with 95% of the time... and that last 5% ends up being the most important to them.