Diff pairs on PCB don't couple to each other very much, with maximum coupling when they're on top of each other in adjacent layers. In that case, the environment seen by one trace is ground plane on one entire side (at whatever the layer-plane distance is), and only most of the other side is the facing trace. So, maybe say 40% of its environment is the neighboring trace, the pair. You need removed ground plane to get higher coupling, which invites common mode issues (that you only want to have to deal with on wired connections where you're saving the cost of a shield, and potential ground loop issues).
Coplanar (side by side, same layer), the cross section is edge-on, which isn't quite as bad as it sounds because the effective thickness is more than the foil thickness -- the electric field fringes out a bit between traces. The result is more like 5-10% coupling.
Which also means coplanar ground doesn't do much: it only reduces Zo by say 5%. It's more important for reducing coupling between traces. Which, is an odd choice here when the signals are complementary, but is handy when sensitive signals are present.
So, diff pairs aren't being used onboard because they're differential: they're primarily not; they're being used because it keeps common mode synchronized between the two traces. That is, they're exposed to identical environments, at the same time (position along the trace). And this works as long as CMR is respected, which should be easy enough: you'd need a terribly nasty board to violate even 1V of CMR.
Tim
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