Actually, you are wrong. Currently active top professionals are almost never available for pro-am in the ordinary sense.

The common names you have heard are those whose careers as competitive contenders are over, or realistically on hold for the moment. They may seem impressive as dancers, but compared to their competition they are not going anywhere. Those who are going somewhere are usually too busy training, travelling, and coaching their successors.

There is an exception to this, which is the handful of cases where a promising but struggling professional has entered into what is really just the old patron(ess)-artist relationship, usually with a single student.