because nobody regards the vibration sensor as sensitive, smartphones typically leave it with wide-open permissions.
What Nirupam Roy and Romit Roy Choudhury did was to hack an Android phone so its vibration sensor acted as a microphone. Well: a vibration sensor is half-way to being a microphone anyhow, in terms of its basic function.
As they note in this paper, “any vibrating object should respond to air vibrations”. What makes a microphone different is that the diaphragm is very light, and therefore responds well to quiet sounds and high frequencies. The vibration sensor, on the other hand, doesn’t respond much to either.
As the pair says in their paper, “VibraPhone is attempting a different problem altogether – instead of learning a motion signature, it attempts to reconstruct the inherent speech content from the low bandwidth, highly distorted output of the vibra-motor.”