He says it's not. And to be fair, he's already loaded so he doesn't need it. But there's bound to be something about the move that is just for the money.
Also he's getting old. He's played for the best club in England (at the time). He's played for one of the best clubs in Europe, alongside some of the worlds best players, it looks like now he wants to make a big name for himself in America, and bring his illustrious career to an end.
He's not going to play International Football for England again, so why not try and do something in America?