A variety of reasons, some others touched on, but here's another: lack of adversity.
Human beings need adversity in their lives just to feel alive; they need some kind of conflict or else they don't even know they exist. Modern society in a first world nation is so damn comfortable, even for many below the official poverty line, that instead of natural adversity (having to till the soil and forage food) they have to seek it out.
A lot of people are able to do this in a healthy way by playing or watching sports, or competing with others in video games, or chess, or really anything where they end up at odds with another person. When this happens, human brain satisfied, and seeks no other conflict.
But for some reason, a not insignificant amount of people just won't do this, and so they create problems where there are none, or simplify complex issues in order to be in direct conflict with others.
I think the reason for this is that narcissists desperately need that higher sense of self so they go this route - and as it's basically instant, as opposed to putting all the time and effort into becoming the best in a specific realm of competition, you see them just come out of the woodwork once such an easy avenue sprouts up.
