It's a case of "internet cool" which is retarded because no one likes those people in real life. It's incredibly annoying as well.
But it can also be a case of hyperbole for the sake of laziness, saying "I hate humanity" and letting people figure out that you can't possibly mean that is a lot easier than saying "I hate T, U, V, W, X, Y and Z groups of people but only these kinds of X and that kind of T etc etc".
But it can also be a case of hyperbole for the sake of laziness, saying "I hate humanity" and letting people figure out that you can't possibly mean that is a lot easier than saying "I hate T, U, V, W, X, Y and Z groups of people but only these kinds of X and that kind of T etc etc".