Because <another communications channel> is the channel that every user already has set up with the user they want to chat with. It varies for every person, so

Because <another communications channel> is the channel that every user already has set up with the user they want to chat with. It varies for every person, so why not let them use what is most convenient for them? Nothing's stopping anybody from writing a walled garden site that provides a nice ui for messaging contacts you stored in an address book on top of that.