I realized this the other day: it's funny that back in those days, you didn't have two computers (or teletypes) close enough together for them to talk to one another just through serial data. Why would two teletypes need to be in the same room? Just use a typewriter! And furthermore, how could you possibly fit two computers in the same room?! You probably had a phone line (or a radio set) handy so two modems would sit between the two devices and handle the long distance portion of the conversation. The null modem cable kinda developed as a thing once it actually made sense to have two computers in the same room that needed to share data faster than you could just enter it yourself.