The main reason is the laser p/u and clock! Dedicated laser p/u and clocks for audio redbook and dvd video are spec'd differently. So though the drawer mechanism of a dvd player can be as sturdy (if not sturdier) the circuit was made to read video data not audio.
As for single disc vs. changer, just think simple is better. However, even a cd changer outperforms a single disc dvd player as a redbook transport.
As for single disc vs. changer, just think simple is better. However, even a cd changer outperforms a single disc dvd player as a redbook transport.