Ugh. At least the world has managed to have "Email" as a standard, a medium, that is not controlled by any huge corporation's grubby claws. Why not with chat? What happened to this? Why is there a war for my data with these big companies, who do NOT have our best interests in mind, and never will? Why can't we have a federated standard for chat, controlled by no one? It seems that we have come close at various times…
Because open-source chat platforms lack features that make apps like WhatsApp good for regular users. How long does it take you to set up a Jabber client? Can you add a person to Jabber using just their phone number?
It also supports adding a new contact by phone number, and replaces the stock text messaging application, automatically upgrading text messages to OTR XMPP sessions if the other party is on the same network.
It doesn't do federation though, as far as I know.