Google is a machine learning company. Knowing about how people use their phone (Android), Computer (ChromeOS), etc a self driving car gives them more and more data to refine and expand their algorithms.

At the same token it is because they have these vast troves of data and some of the best machine learning scientists in the word, they are suited to do this type of research.

Another thing to think about is that if we are not distracted by the commute we will spend more time using Google services.