[vbrandl]

I would guess OSM uses optimizations for eucledian graphs, where the path a->c is always shorter than a->b->c. This restriction makes e.g. TSP solvable. But this property does not hold for any generic graph. I don't know if this makes visualisation also easier.

[michaelt]

Technically, if you've got a bumpy dirt track a->c and a freeway a->b->c then the travel time on the latter route can be shorter.

Of course, they do get to dodge a major problem: That high-dimensional data is hard to visualise in an understandable way. Everyone knows what a map looks like, nobody knows what a clear visualisation of a set of 100-dimensional vectors looks like.