The reason they become leftists is that they are shocked by the inequality/injustice/suffering of the world. And the reason they are shocked by this, is their humanist/liberal upbringing. That's the point, it is reproducing itself. Even if you're not brainwashed (politically) by your liberal parents, you're gonna become a leftist during your adolescence years just because the world out there is not as "good" as you've been taught by the garbage tv shows for kids and Christian stories of love and equality.
Suddenly, problems arise, the intelligent ones can perceive them all, the mediocre ones can perceive a few: Anything has a price tag on it so what matters more is marketing and not substance, spiritual health and virility (for men) are TOTALLY ignored (in favor of getting technical skills), most girls are whores, even more men are fucking morons and sexually dependent worthless drones, social skills are on a free fall, the nerd/loser caste is expanding expotentially (it will try to devour you if you are not alpha enough) and it's THEM, OR the hipsters OR the status-hungry types. And of course there are the big scale problems, ecocide is coming, economy is crashing, something is wrong with the third world, culture is non-existent in the West, race war, cultural war, criminality, drugs, depression, sexual perversion etc etc.
If you are a fucking moron you can blame, what you perceive of all this, to capitalism and become a leftist. Or blame it to capitalism/intolerance/lack of humanism and become a moral liberal, or something like that, categories are a bit different in Europe.
Anyway.. there are some people who play it leftist to gain social power but I would call them manipulators and I believe there are the minority. There are also a lot of leftists who might have the morality of leftism but cannot apply it in all situations (since it is surreal in itself). For example you might be a leftist and believe in integration, but, let's be honest, you cannot buy a house in a black neighborhood and send your blond little daughter to a school there, you just can't... instinctively. To a degree, I can understand how these pathetic morons think.