It does give you the option to change cards when you double tap, before you pay. But I think the real reason is that people would walk around with card readers

It does give you the option to change cards when you double tap, before you pay. But I think the real reason is that people would walk around with card readers and scan peoples watches to collect cash. I can only guess it doesn't happen in Japan for cultural reasons. You could try and do that with purses/wallets too, but since most people are carrying multiple cards, they interfere and largely prevent RFID skimming.