I might be missing something here but my thinking is that Google Inc/Alphabet is bigger than Larry Page. If for whatever reason he was not able to attend he could have sent a representative chosen from among the senior executives. It seems that neither his attendance nor Google's participation in the whole thing was not mandatory
According to the article they did offer to send someone else. Presumable the senate was not satisfied with anyone other than the top guy.
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.