Given that Google and Apple are both Silicon Valley tech firms, it is likely that engineers who work at Google will also one day work in Apple and vice versa. Engineers moving around ensure similar ideas are spread. Of course Google and Apple will copy each other.
While I agree that the two operating systems have similar appearances, there are still key differences between the two operating systems. For one, Google is primarily a software company that makes 93% of its revenue from advertising. They Google produce a phone, tablet, or computer, e.g. the Nexus 7, they make no profit on the product, aiming instead to move the market forward, get the device into more people's hands, and make money off advertising. Apple, on the other hand, makes most of its money off the hardware, e.g. making the iPhone 5 for $200 and selling it for around $700 to $800.