Simply because I can operate all non-pactor digital modes with a sound card, using one end user program or another, and I don't want to tie up another computer port, save/maintain another configuration, and spend money (little/much is relative) on something only required by some software, but not all.
FWIW, I've never had any trouble with AGW, albeit I do use Packet Engine Pro which has a somewhat more capable interface than the free version.