[croon]

1) There are different speed limits on different roads. If you put an artificial limit on how fast your car could go, it would still go illegally fast on slower roads.

2) Speed limits change. It would be a hassle (understatement of the week) to update every single car.

3) As others have said, you can always drive legally on private tracks.

[soco]

You could nowadays easily imagine a GPS-based governor according to the speed limit of the road you're on...

[croon]

Sure, and I can also easily imagine many ways in which it fails :)