He started boxing when he lived in New York. Robinson is about as much (actually, less) of a Michigan fighter as Joe Frazier is a South Carolina fighter, or Joe Louis an Alabama fighter.
Harold Johnson, Billy Conn, Harry Greb, Tim Witherspoon, Matthew Saad Muhammad, Jeff Chandler, and Charley Burley are all Pennsylvania fighters too.
On the question of what state produces the best, I'd go with Pennsylvania. Not just quality but quantitiy too.
I thought I put James Toney in there, since I'm adding Toney, might as well add Leo Nolan, Rydell Booker, Stanley Longstreet , Bernard Hayes, Micky Goodwin, Bronco McCart and Tony Tucker. Hilmer Kenty is from Cincy, but he moved to Detroit and still lives there.
Michigan, Missouri (STL where I'm from is a boxing city, you almost always know somebody who either trained or boxed before if you grew up there including myself) New York (where I live) and California.
Per capita Massachusetts has to be up there. Off the top of my head: Rocky Marciano Marvin Hagler Sandy Saddler Mickey Ward Jose Antonio Rivera John L Sullivan Ray Oliviera John Ruiz Robbie Sims Freddie Roach Dana Rosenblatt Sam Langford Edwin Rodriguez As far as what region has produced the best it's definitely the northeast, from Penn. to New England.