For some it is because the publishers take the copyright. For others it is just because the authors don't really care, don't know about it, or don't really think about it.

I am a professor and I was at a meeting a couple weeks ago that talked about open access publishing. There were people from many departments and there was one professor there who asking questions that made it clear he didn't know anything about online publishing (he was asking things like "where are the papers stored?" and said something about how "all this online stuff is like Big Brother")