To me, this is because we are still not in control of our lives. Davies says as much when he tells us what superstition is about. He says it is about trying to make sure we have some control of our fate.
We may know a lot more about things today, but we certainly don't know everything. In addition, some stuff is still, as far as we know, due only to luck. Why does one person get cancer and one person not? Why is one person in the way of the drunk driver and not another person. Both of these are at least partly luck.
So as long as luck rules parts of our lives, there will be some people who are superstitious.