It already is that, which is precisely why google is using it here. Google is an american private company. Phone numbers have government mandated systems around

It already is that, which is precisely why google is using it here. Google is an american private company. Phone numbers have government mandated systems around the world that allow a individual to keep using them even when they lose their phone. Google uses it because it lets governments solve the identity problem in the fashion and to the degree they deem acceptable, and leaves google in the tech business. Some countries have issued ID cards which support encrypting and signing documents. If that becomes more widely practiced, then google could switch to that instead, but until then I imagine they'll keep using phone numbers.