I was losing my mind over this for days! It looks like there is a bug where if the osm.pbf is too big (too many nodes) to successfully build a network, it doesn't give an error message -- it just returns "the network.dat file has been successfully created" anyway.

When I ran the same code with an osm.pbf with few enough nodes (in my case I clipped my state OSM to a bounding box around one city), the network.dat file showed up in my working directory.