The Google Voice app never purported to be a VoIP solution. It's simply a UI to the Google Voice API. When you attempt to make an actual call with it, it sends you off to the built-in dialer with a "predialed" phone number. The calls you send and receive via Google Voice use your cellular talk plan and not the data plan (which VoIP would do.)

tl;dr- GV is not a VoIP service; it's a call management service

Talkatone still exists to this day allowing for Google Talk functionality up to, and including, Google Voice calls over 3G.