Because it's graphine and the entire point is that google is not involved in any transaction between you and your banking app.
The transaction isn't between you and your banking app. It's between your bank and a load of proprietary Google shitware that Google cajoled Visa and MasterCard into accepting as a replacement for an actual card.
The transaction isn't between you and your banking app. It's between your bank and a load of proprietary Google shitware that Google cajoled Visa and MasterCard into accepting as a replacement for an actual card.