Why can't people on the internet spell "masturbation" correctly?

You'd think that as much as they do it, the basement dwellers that lurk the internet should know how to spell it. On Y!A I've seen people spell it correctly in the question, but then some idiot comes along and answers the question with the misspelling "masterbation." They even underline it in red so you know it's wrong. Why are people so stupid?