the mic produce not only the signal from the capsule but also self noise from the electronics so when you amplify the the signal you also amplify the noise .
in situation when you have poor signal and need to rise the levels eventually the self noise become notible by your ear , usuallly that not happenes as it masquerades by the ambient noise and the preamp self noise
but in the desert in complete silence with mic 2 meters over the mumbling actor you may hear it -