Keep in mind the phone's primary interfaces used to be the keypad, speaker, and microphone. They weren't always general purpose computers. You dialed a number, and then spoke to that person.
Keep in mind that the phone's primary interface wasn't always a keypad, it used to be a dial or a lever. The speaker and microphone were distinct parts rather than a single "receiver". And they all slowly converged to a couple of very similar optimized designs.
It's the manufactured equivalent of carcinization.