>If that is true, why is it true? I think that's a far more interesting question.

Almost any scientific finding should be ended 'but more research is needed to be certain'. Perhaps the question to ask is why isn't this getting more research, given just how important the topic is to the lives of so many people. All of the questions you raise can be tested with further research, so why aren't they? Why is some of the best data (best is not to be confused with good, as best is relative) from dating web sites?