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Because they spend their time on (mostly online) communities of like-minded people who display similar childish behavior and praise each other for it. In real life offline communities, they'd usually get told to "get their shit together" and be bullied into submission. Unfortunately, since most people just carry their cellphones around all day long and can access the Internet practically anywhere, they don't feel the need to socialize with people physically around them if they can just chat with strangers online.
In many large cities you'd usually find an anime/gaming club over at the city's university or shopping arcade, but even though they often have people who are kind of odd, they're almost never anywhere near as odd as the kind of people who unashamedly post about their sexual fantasies and spout their political views without any restraint here on 4chan.