Same reason we don't zombie-proof our houses. There are plenty of things to worry about that actually have the potential to happen. Why waste time worrying about things that won't?
I think it's easy enough to develop an accessible site, then progressively enhance it. The benefits of having it well written in that way has more benefits than just pure accessibility (SEO goodness comes to mind).<p>In certain sectors, accessible sites is mandatory already.
Also it's easier to create browser technology to change the appearance of sites to match an individual's needs rather than all websites to change to meet a generalized set of accessibility guidelines which may not affect all disabled people and which may harm abled users.