data signal on headphone jack input on iPhone
What you're missing is that the microphone input on the phone is not just a bare ADC.  It's intended for receiving audio signals, which means it has isolation, DC offset removal, filtering, amplification, normalization, etc. then an ADC.  Microcontrollers are used to modulate the raw sensor input into a signal that looks like audio, because an unmodulated DC signal wouldn't get through.  

Of course, you could do the modulation the old-school way without a microcontroller.  A microcontroller just happens to be convenient since it can be used for both analog and digital sensor data.