Because even google doesnt support the voice chat features of googletalk on any OS but windows.
the client is win only, you can use the 'gadget' which is done via a browser (and quite well implemented i thought) for messages, but not voice chat (it tells you it wont work when you try).
i was reading something earlier about how they released the jingle libraries so it may not be long.
Gizmo is supposed to support it in some sense (not sure how you interact with google accounts though).
or you can try ekiga for SIP calls.
it's a shame really, cause Skype is a complete load of wank, and google talk doesn't support the OS