it would cost to produce those cards! those costs would be passed on to the likes of me and you. it would mean vast majority of folks will have to pay for the service that a very few people would use, so it does not feel fair.
one option is for GG to pass the costs only to those that use this. so for example getting the swipe card would costs an said amount to cover the costs for GG to run the facility.
there are also options to top up via paypal, credit or debit card where you do not need a voucher. Even with a voucher it only take less than a minute to top up so not sure the value that the swipe cards will offer.