I was the one who converted Google's CLA system from "Please fax us this form" to a web form. At the time (2007, 2008?) I added all the fields from the fax template to the web form. But we dropped the phone number & physical address years ago. And they were optional even before that.

Nowadays we ask for your name, email address, and optional GitHub username.

Probably to keep a file of all contributors' contact information to get your permission if they ever wanted to do a license change?

Because Go is owned and controlled by Google, the company famous for personal data collection and misuse.