Technically: it’s a function call that must return text. Imagine if you sat down at the command line and typed an initial command, then from that moment on every response required you to issue a new command. ping-pong-ping-pong on and on and on “forever.” There isn’t a choice to walk away and take a nap. Text in must result in text out. Eventually, given enough time, it might have devolved into outputting shockingly coherent poetry about ferrets, but in the mean time there was still a lot more valid combinations of technical explanations and commands.

Morally: Not applicable, see above.