Why is Google Voice still such an outsider on Android?

Seriously. I can make and receive regular phone calls over WiFi on Voice in the browser on my Mac and Windows machines. Why on earth can't I do that on a phone designed by Google? I've got a SIP gateway set up, and it basically works (as does the Vonage app), but it just seems downright absurd that there isn't OS-level GV integration. T-Mobile loves to tout WiFi calling as a feature on certain models. Why wouldn't Google want to be the first to bring that to an entire platform, regardless of region or carrier?