The fact that TeamSpeak / Skype already has a ton of features, and re-implementing them all in Blockland would be hell. And if you don't implement some of these features, you're missing some nice features. Plus the fact that Blockland simply wasn't made for voice chatting, while the previously named programs were, and would do a hell of a lot better job at it in all cases.
In any case, you're not going to be changing my mind, and I won't be changing yours, so arguing like this is kind of pointless.