Because only the outside part moves if you have the inner pad set too far off the disk then as you operate the brake you can see an alarming (relatively) amount of disk bending which isn’t good for noise, vibration or wear. So I just set up the inner fixed pad so it just kissed the disk which doesn’t produce any rubbing noise if your disk is straight, and doesn’t drag, and means you minimise the amount of disk bend when you operate the brakes. It’s the best compromise I’ve found.
But either way it’s not a great design or a great solution and you might get it dialled in, but in a few months you’ll be having to repeat the process. Best to just bite the bullet and make the investment in a hydraulic brake solution.
A work colleague of mine ditched his cable disk brakes and put on the Giant cable operated hydraulic set up and he thinks it is a massive improvement and wasn’t as expensive as replacing the whole brake and shifter setup.