No one can tell you why anyone does anything. You haven't said what sort of arrangement you had with him regarding paying him for the time and effort he expended on your behalf, so it's impossible to tell if the attorney in question acted properly or not.
You have no basis for saying that "so many attorneys do this." Have you had dealings with multitudes of lawyers? What's your experience with members of the legal profession?
If you feel that you have been injured by the attorney's actions, contact the Grievance Committee for the Appellate Division, First Department.
I am an attorney admitted solely in NY. None of the answers I submit on this forum constitutes legal advice, even to questioners in NY, and no attorney-client relationship is hereby created.