re: How/why are leftists so sure of their beliefs

Arrogance. They believe the classes they took in college make them smarter than you, therefore their ideas are correct and yours are wrong. They believe anyone who disagrees with them are uneducated and are backwards thinking.

Plus, they believe that the media isnt corrupt and that there isnt some big "conspiracy" out there trying to take away our freedoms.

Basically, the left are not capable of deep thought or critical reasoning. They do not question their own beliefs, ever.