Primarily, people buy into the mainstream propaganda worldview because humans are lazy.

By this I don't mean to say that people don't work hard enough or don't stay busy enough; humans sleep less than any other primate on earth, and if anything the world would probably be better off if our species chilled out a bit. When I say people are lazy, I mean we are lazy thinkers.

And we are lazy thinkers for reasons that aren't really our fault. The human brain is wired to select for cognitive ease, which means we tend to favor pathways of thought which require less mental strain in order to conserve energy — probably because our evolutionary ancestors needed all their mental energy for important stuff like finding food and avoiding saber-toothed tigers. 

If that wasn't bad enough, our minds are also wired to preserve our existing worldview, so that the perspectives we form from our lazy preference for cognitive ease are held in place, and evidence which contradicts them will often be rejected. This is why facts don't tend to change people's minds.

This lazy tendency to select for cognitive ease and defend the worldviews we construct as a result of that tendency is what gives rise to confirmation bias, because believing things which confirm our preexisting ideas about the world is easier than believing things which would blow our worldview apart. 

If you're among those who've gone from fully believing the mainstream propaganda worldview to realizing that everything you've been trained to believe about the world is a lie, then you know how uncomfortable and disruptive this shift can be. Our psyches are stacked toward avoiding that work and discomfort, in the same way they're stacked against exercising regularly even though we know it's good for us?—?which by the way also happens because there was a time when conserving energy was beneficial for our species' survival, causing an adapted preference for rest over exertion.