Google might not have your credit card, but it has the credit cards of 500 million people that have Android phones and purchase apps[1]. I think that's a pretty sizable chunk of the population. Being able to use Google to buy things (and digital things) is huge. Yet Paypal still rules as of today.

[1]: https://plus.google.com/u/0/+HugoBarra/posts/R5YdRRyeTHM