Most people traveling North on I-95 don't go anywhere near Philly. If you're coming through Delaware North toward NYC, you take the Del Mem Br over to the New Jersey Turnpike (which becomes I-95) rather than take I-95 on its roundabout spur towards Philly.
Right. The route you describe is the fastest route between Washington D.C. and New York. The freeway through Philadelphia was designated I-95 for political reasons -- because some influential member in Congress decades ago decided that I-95 would have to pass through Pennsylvania.