[nathan_compton]

Arguably its because in many cases mass delusion is adaptive - it organizes and simplifies how groups of people behave. Religion is a cultural mass delusion (don't worry, I'm not talking about YOUR religion, just the other ones) with significant adaptive qualities, people argue.

Even more broadly, rationality and empiricism are extremely powerful tools for understanding the world, but they are also extremely difficult and costly to use, to say nothing of their known limitations at tackling complex systems. People have to and do believe stuff just to get through their day. I think this only becomes disturbing when its just one part of the culture which decides to believe some other complex of ideas than the mainstream one.