It seems obvious to me that you're not an 'engineer'. :-)
The microphone manufacturers are well aware of this phenomena. Indeed, to the extent that a major design goal is to _minimise_ diaphragm mass to reduce the unwanted effect of external mechanical vibration producing an output signal in competition with the air borne sound waves they're designed to detect.
However, in this particular usage case, it would be most useful to enhance this otherwise undesired effect by increasing diaphragm mass to enhance detection of the vibrations injected into the body of the microphone itself. The extra mass in this case acting as a 'counterpoise' to magnify the relative movement of the diaphragm with respect to its mounting. It's simply "Mechanics 101".
However, if the piezo transducer is being tasked to both emit and detect vibrations as in the case of my own electronic scales (it only beeps to indicate it has finished measuring btw) then it'll remain unmodified to avoid compromising its performance as a sound emitter and its inherent shortcomings as a microphone utilised 'as is'.
Since the manufacturers of my particular model of bathroom scales manage to successfully detect the 'wake up toe tap' signal without such embellishment to the transducer, economies of scale in production will apply equally well even for those models which don't require it to be used to provide audible feedback as well (thus answering my musings on the absence of such mass enhancement to the piezo transducer). The inherent shortcomings of the piezo sounder as a microphone is sufficient of itself to nicely serve this purpose.