Except that wasn’t a serial port, it was midi, and the reason it was on the sound card was because the input was analog.
Your joystick was just two fancy potentiometers, and your soundcard decoded the voltage on the middle legs into a position.
Soundcards handled joysticks because they had the fastest ADCs.




::: NSFW, but I can’t mark a picture or comment NSFW
And here I thought I had it all figured out. But it does make sense. Doing it with an analog signal introduces noise and measuring pulse widths is going to be simpler.