Some people – you’re born, you know, the wrong names, wrong parents. I mean, that happens. You call yourself what you want to call yourself. This is the land of the free.
(CBS interview with Ed Bradley)
As far as I can tell, the vast majority of people use Internet nicks, rather than real-life names. Reasons for this might be various, with anonymity possibly the primary one. For me, there’s one even more important: I hate my real-life name. The forename has no cognate in any other language and features a phone which occurs neither in English, Scots, nor Gaelic. The surname has sound combinations unknown in these languages – and a letter you can only type on an English keyboard using HTML or Unicode. So in addition to my Thrissel nick I also use the ‘substitute’ full name of Keith Robertson.
Why don’t I change my real-life name? Would that I could. This is not the land of the free. You’re only allowed to do that if you convince a registrar that your current one poses a great problem to you, for example if it strongly resembles a taboo word. But even then the decision isn’t up to you, it’s up to the official.