[enneff]

Anything you can study up for in a couple of weeks is also something you can learn on the job. What's the point of throwing a perfectly good candidate into an unfamiliar situation without any prep? When you actually get the job, there are plenty of people around to help you learn new skills.

[Bahamut]

On the flipside, Google probably wants its employees to be motivated to self-learn/develop. It also allows the candidates to have a certain baseline of knowledge before having to pick up Google-specific stuff that they'll probably be flooded with.

Google has a strong reputation for paying well & being a desirable company to work for in the minds of many, so they can afford to do this as a part of their process.