I found on the Garmin if you set "avoidances" to Interstates and Major Highways it'll take you down all sorts of interesting little roads. Much slower if you're going somewhere of course, and it doesn't actually take you off piste should you be in an open desolate terrain. But if you head out on a dirt track or across country at least the satnav will keep indicating the general direction you want to head, just combine that with a bit of dead reckoning.
But I think Vaufi hit the nail on the head: most travellers are going between A&B and will tend to gravitate towards paved or at least recognised routes. It may not be easy to find dirt tracks especially in the developed world. It's the difference between travelling and just taking your time and exploring an area. Not all of us have the time, money or inclination for the latter.
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