Why are people stupid enough to believe propaganda?

Well, some people just aren't educated enough in certain topics, so the first thing they hear about those topics are what forms their opinion. And then again some people are just too lazy or too careless to know what's really going on, that they rely on others to tell them what's going on, and usually that spells trouble. After that, people search for things that support their opinion, whether it's true or not. Like when some people say that Muslims want to destroy all non-Muslims; they'll believe it because they already have a bad opinion about Muslims, and this other opinion only furthers that idea.