Because I don't necessarily feel my face is a contributing factor to discussion on a board centered around videogames. Also, there's a lot of personal, somewhat intimate stuff that gets talked about on boards like these and I don't want to feel hindered in discussing these topics just because my face will irreversibly be attached to that statement. I feel the recent backlash against prominent folks in media/entertainment is cause enough to prefer to stay anonymous even if I will never in my life willingly make a racist, sexist or misogynistic comment.
Also, I prefer to keep a low profile on the internet in general. I don't need people finding my messageboard posts from a decade ago when they google my name or reverse image search my face. I deleted Facebook for that very reason and I don't use or read Twitter.
Say that this board at one point gets exposed for having some sinister folks on it, or pedophilia charges or what-have-you. I'd prefer to have a barrier between me and direct association with these people.
But I do try to stay accountable for what I say and/or do on a forum like this. If something I say is wrong and someone points it out, i'll own up to it without rocketing into defense-mode. So in that regard I already feel i'm doing my part on making this forum a bit more hospitable.