This is a legacy of wiki-syntax, where all external links have to begin with the protocol. It would be nice to make the VE link inspector smarter than this.
"www.google.com" is almost certainly meant to be an external link - if there is an article by this name, then it could be shown in the link inspector dropdown as well, but the option for "External link" definitely needs to be there. Even something like "royal.gov.uk" should be recognised as a URL.
Requiring "http://" or some other protocol at the start is confusing for the newbie, as the protocol component of URLs is generally ignored nowadays.