Why? You're completely ignoring the demand side.

Weaker JPY means lower demand for Japan-originating travel. It's far more than forex differences though, as you have to look at the relative strengths of the economies, spending trends, etc. US-origin airfare has been dramatically higher than basically any other market for over a decade now, because Americans can and will pay more.

More than any other line of business, commercial airlines rely on (and exist only because of) an ability to get different customers to pay different amounts for the same product based on their ability to pay.