This is efficient in a digital medium, where scrolling down is easy and unbounded: not a waste of pixels the way it might be a waste of paper. Linebreaks come for free: they don’t take up any more bytes than a period and a space, and they add a lot in readability. Both “new line” and “send message” take a single keystroke, often the same enter key, so the muscle memory is easy. Plus, it helps the conversation flow better if you hit “send” after every utterance rather than waiting and sending a whole essay: the reader can start thinking of a reply sooner.
