Publishing: Why are we doing it right and other sciences aren't?

(NOTE- this is NOT a `we hate Elsevier and the others' post- though I suspect the comments will be about that.) Alexandra Elbakyan has created a repository of science papers for scientists to share. She did this because too many papers were behind paywalls. An article about her (and the inspiration for this post) is here. Note that she has had some legal problems. But it got me thinking: I have rarely had a problem getting an article. Between authors websites and arXiv most of what I want is out there.  Lance and others I've asked agree--- while there are problems with editors and publishes etc, access is NOT one of them. Why don't other scientists just routinely do this? some speculation but if someone actually knows please comment 1) Some scientists purposely don't post on line to avoid being scooped. Or the fear of that 2) Some scientists don't post for fear of legal issues with publishing. I do not know of a single computer scientist or mathematician that has gotten in any kind of trouble for posting a paper on their website or arXiv. 3) Some scientists are lazy. 4) (This contrasts with Comp Sci) other fields do things the way they do since they ALWAYS did it that way. Comp Sci is a younger field and hence is less tied to tradition. 5) Other scientists ARE posting their papers (gee, then why did Alexandra have to create the reporistory). 6) Alexandar is where arxiv was X years ago. They'll catch up. But they are having legal problems-- did arXiv ever have legal problems? Looking over these I'm not sure I believe any of them. Any other ideas? Again, the question is: Why don't other fields routinely post to something like arXiv or their own websites?