Because I can.
You can't stick a sign on everyone's keyboard saying "Leave your preexisting social constructions and schema at the door!" Culture, which is the real issue under discussion, is so ingrained into our very being that the idea that we can just set it aside and walk away is almost laughable. Even on the internet, where physical appearance is removed from the first impression on most forums, culture is inescapable.
This shouldn't be a terribly surprising fact. Communication is inherently a cultural act. Every comment is made in a cultural context, and it has been suggested (in a lit. crit work that I can't find a link for now), that without shared context, communication is impossible regardless of linguistic difficulties.
So, if we cannot leave our culture behind when we log on, does this mean that the Internet is a cultural dead-end? Are we doomed to remain stuck in last century's cultures forever?
No!
Culture is emerging in communities across the internet. One particular example that I'm a member of, a community which exists around a shared continuum of stories, provides a very good example. Though jump-started by revolving around a fictional continuum, we are beginning to generate our own culture. We have our own social structures, our own legendary figures (both fictional and real), and our own shared language which is approaching the incomprehensibility of the English spoken by biologists or computer scientists.
This is the true strength of the internet as a cultural machine. Old cultures will not magically reform or disappear, but the internet allows the creation of new cultures, based on interests and hobbies rather than geographical location. To paraphrase Randal Munroe (who draws a hilarious webcomic), I look forward to the day when "where are you from" can be answered with "the internet", and people ask "what part?" instead of "no, seriously."