there has been a few standard units mp3car had made one, lilliput and a few other chinese manufacture has made them. In my opinion they're not popular for the following reasons:
1) You can't fit mini itx into a double din slot easily, and when u can its so cramped that you get into cooling problems along with fitting other components in. Most of them have some problem/limitation that can easily overcome by non-double din unit.
2) Most of them are nano itx and that comes with two problem: they've slow and they're expensive.
3) Mostly people that want a carputer are hobbyist and we want to build stuff ourself
4) The only way these things would be widely popular like the normal double din units is if you got a company with lots of resources like apple/microsoft that puts a lot of money into designing a unit that is fully environmentally compatible and mass produce them in large scale to keep their prices down.