Just because a lot of people do something, that doesn’t make it safe. Drinking alcohol is harmful, but accepted by a lot of people.
The argument against all magical thinking is a bit of a slippery slope argument, so it’s not perfect. That being said, do you really think most people actually believe knocking on wood will prevent something bad from happening? If someone truly believes that their superstitious rituals have an effect on objective reality, then yes, that is very harmful. If someone bases their beliefs on something other than evidence, that person is not only unequipped to make good decisions, but also an easy mark that can be taken advantage of.