It's due to fraud risk. If googles threshold billing is X, then their risk of revenue loss is X*(N+1) where N is the number of virtual card numbers that have been created for a single physical card. The +1 is for the physical card.

Visa/Mastercard likely don't supply a way to link a physical card to it's virtual card generations (that'd be a security risk), so Google doesn't know that virtual card A is associated with physical card B.