Because with "new" Google Pay, all of those membership cards are gone. They didn't just not migrate the data over; this isn't a feature present in the new one. I don't see any reason to continue using it. Apple Pay is working better for me for tap-to-pay, and I'm instead trying to replicate some of the old membership card organization with Apple Wallet, albeit without much success yet.
(Can't believe we have to use "old"/"new" delimiters here either, while Google gets to pretend it's the same thing.)