In part because Google may wish to do something that required that money at some point in the future (for instance an acquisition or purchase of a chunk of pate

In part because Google may wish to do something that required that money at some point in the future (for instance an acquisition or purchase of a chunk of patents to think of just two multi-billion dollar expenditures they've made in the relatively recent past). Giving money back to shareholders is relatively easy, raising it from them if you need it for something is more involved. And if you're going to hang on to it in some relatively liquid form, you might want to try and get a better return than you would from sitting on a pile of cash.