Why, because I don't think I have ever gone to my profile page, ever, to look at the settings.
I only have a profile so I can respond to threads like this and express how petty a person must be to complain about something like this, as a plague.
Excuse me. a 'Terrible plague'. Rather dramatic, don't you think? Stop worrying so much about why or why not I care what my online persona is, and start worrying more about why this is so unsettling to you that you feel you need to post about it. I feel you may have a void of things to complain about in life, so instead of being happy, you need to make up new crap to complain about. Either that or you get some type of pleasure out of pointing out how superior you are to us unenlightened folks who don't give a crap how big our e-penis level has gotten.
And to answer your question, as to why I keep the default. Cuz i'm a lazy person, who can't be bothered to change it, and the default avatar 'snapshots my personality' by showing you just how lazy I am.