Couldn't a person just download a piece of free software and do
the same thing with the audio card that's already in their PC?

I bet they could.

Unless they also have a DSP audio processing program, they would not have the
quality filtering found in these commercial-grade units. Also, I believe the
number of modes is greater than any I looked at. And, finally, running a
software/soundacrd program may prevent other programs from running
simultaneously on some PCs. You might be right, or maybe not. There are sure
large number of dedicated external decoders still on the market: wonder why?