Are we trying to make this an either/or instead of both/and?
Of course they're going to make a bunch more money here, but the money is here because (to some extent) that's where the talent is. These are insanely gifted and competitive athletes. We can debate how much of the decision making process is "I want to compete against the best" vs "I want a bajillion more dollars than I can make in Japan," (and that's going to vary by player), but the former is absolutely a factor here. It seems like you're saying it's not, and I think you're wrong.