....so why is it not preferable to have a computer in a stereo system?
IMO, the answer depends on the level of audio nirvana you're wishing to achieve. Meaning, while a computer USB output to an external DAC USB input is very doable, listenable, etc., it is often considered (by the DAC manufacturer) the least quality with respect to other inputs the same DAC would typically have. Also IMO, I believe the same can be said of the USB cable with respect to S/PDIF, AES/EBU, etc. cables. The aforementioned said, the difference could still amount to splitting hairs and you may never realize or care to realize there are better options than USB. IMO, if USB works for you and this is your way of keeping things simple, go for it.