I'll preface this with noting it's a little odd to act as though adding 80 MB of unnecessary JS is a popular idea on HN.
They couldn't if they needed to work asynchronously. Or if they couldn't speak or hear well over the phone, the likelihood of which approaches 1 if you have so much as a different accent. Or if you don't want the other person to hear your accent.
It's a bit cliche maybe, but I know people who've benefited from all of those features of IP-based communication, and they're reasons to be happy about the Internet. And since just about any telecommunications system that transmits voice can be bullied into transmitting IP packets, there's really little cost.