I have been looking into this subject for awhile now. Here is what I have found both from direct experience and research.

1. Most low end wireless network vendors do not have WEP enabled as a default.
a. Believe it or not but the Mircosoft wireless products do have WEP enabled as default.

2. When the price of wireless hardware dropped significantly, home users bought the stuff in droves. I have seen a significant increase in wireless activity in my area and most of the time, the networks that are not WEP enable, are from vendors that don't make this a default.

Perhaps the problem is basic human nature. We know it does not take much to enable WEP but since it creates another few steps in the install process, (or after install) perhaps people just don't set ip up becuase they are lazy. Perhaps it is from a lack of education and experience on networks that cause these users to leave WEP disabled. I know the documentation for these devices discusses WEP and how to set it up.

Just my two cents.