I am more prone to resorting to basic physics.
The charge on the electron sets a lower level on the noise floor. It can do this in different ways in different circuits, but it’s still bleeping hard to get beyond 20 bits, even assuming +-10 is “maximum” input (or output).
Likewise, the AIR has a minimum SNR. The noise level of the air at your ear drum (assuming normal atmosphere, both sides) is between 6 and 8.5dB SPL of white noise. This is just BARELY below the actual threshold of hearing, interestingly. So getting from noise floor to 120dB (which is way beyond any sound anyone should ever listen to) is 19 bits in any case, with the air doing the dithering for a microphone with an eardrum-sized diaphragm.
A larger diaphragm will have more noise, but even MORE signal, so you can get better SNR, of course, by making the SIGNAL even bigger.
And then there’s shot noise in the mike circuit. How many milliamps does it take to get a noise floor to peak ouput of 144 dB? Figure it out, I have. 😀