Kent Walker doesn't make decisions about how Google conducts business, Kent's job is to whitewash it for public consumption. I believe his current title is "Vice President of Global Affairs", which is more or less a PR title, and of course, as others have noted, he's their head lawyer.
Anyone can look up Kent's numerous writings on Google's Public Policy blog, the Senate wanted to speak to the people who actually make those decisions, not just be informed what the decisions are.
Google offered Walker, who was just promoted to the position back in July, and has no operational control over the company.
[0] https://www.washingtonpost.com/news/the-fix/wp/2017/11/01/fo...
Source: I used to work at Google. I would have asked Kent a policy question well before Larry or Sergey.