Imagine new aircraft instrument was invented. It only told you which way you were facing not the direction you were going. Relative to a grid system that varies over the earth, as long as you are not turning, climbing, or accelerating. The placement of other things near it rendered its readings useless and could be affected by the currents when other equipment was turned off and on. Would you pay good money to have it installed in your aircraft?
...makes me wonder why a compass is a required instrument and a GPS is not.