Because however poor it is, people still leave the course with more knowledge than they began it with. They still end up knowing much more than the typical gym-goer. And I'm fairly sure most of those who passed would know more than you. About powerlifts, probably not - but there's more to PT than squat, bench, and deadlift. I'm sure I mentioned some examples before.
And as I said, an interested person can use the course to learn things not covered by the course, because they're meeting with people from the industry, etc.
You have to realise that most people come and go from gyms every few months anyway, or change routines every few weeks. They don't hang around long enough to find out if anything actually works.
And for many people, their workout is not actually a means to an end of physique change, improved performance, etc - they just enjoy being at the gym cycling or doing curls and talking with their mates, and being able to tell their friends they work out. It's social, not physical.