There's also a habit in web development of not testing with assistive technologies. I'm certainly personally guilty. Of course, few people test their site's appearance through colorblindness simulators and other things that reproduce how a non-trivial percent of their viewers see the site (yet still bitch and moan about the 2% or so using IE6/7; some 8% of males have a form of colorblindness according to Wikipedia)

Part of the problem is that when assistive technology breaks, it's user is completely SOL. Another is that replacing it is often expensive (both financially and operationally). This makes it a lot harder than simply upgrading to the latest version of $browser.

edit: a comment below suggests that both of our comments are based on wrong info, and that screen readers and such actually prefer nested labels over id/for attributes. I don't have evidence either way, but I think both developers of websites and of AT are doing the handicapped a disservice as fellow humans.