Google's biggest revenue stream is ads. It has many others, but that's the big one. Ads are seen by people who use Google search, and who browse many ad-supported websites. Google also has a lot of users of its services, e.g., gmail, who have accounts. Some also entrust valuable data to Google.
Google therefore has a great deal of incentive to make sure that: (a) Nobody messes with the ability of general users to browse the Internet safely (preserving ad revenue); and (b) It's very hard to compromise users computers and gain access to their Google/gmail/whatever accounts (reducing support costs and keeping users happy), or destroy their data, or exfiltrate it. Even if the compromise was the user's computer, it's still a very bad experience.
Project zero is a pretty good example of this incentive structure in action. (such as fuzzing for windows font bugs: https://googleprojectzero.blogspot.com/2016/07/a-year-of-win... ).
With fuzzing itself, as DannyBee alluded to below, one of the leads of a lot of this infrastructure, Kostya Serebryany, is also personally passionate about seeing it be taken up to make the software of the world better. I've seen this in action - he convinced me to use libFuzzer to improve TensorFlow's robustness, and mentioned that he was doing so in part so I'd take that experience back to Carnegie Mellon and spread the word. :) (and it worked - https://github.com/tensorflow/tensorflow/commit/7231d01fcb2c... for example).