Having enough gain in a source component is not all that is necessary to get the best sound out of that component. The output stage of a good pre-amp is engineered to drive the input stage of a power amp, while any given DAC or phono stage may or may not be.
In a related topic, it is for the same reason that many listeners (no pun intended---see below ;-) find the sound of a passive pre-"amp" to lack something (see Art Dudley's column in the current Stereophile).
In a related topic, it is for the same reason that many listeners (no pun intended---see below ;-) find the sound of a passive pre-"amp" to lack something (see Art Dudley's column in the current Stereophile).