The layout is different in philly. Train stairs are on the sides of the main hall, not in the center of it. The center of the main hall is clear of seats to allow traffic to come through. The design of moynihan puts the stairs to the tracks in the middle of the hall, not the sides of the hall. So the seating options are put near the side to allow more flow of traffic in the center of the hall. I'm also pretty sure this is also a result of modern anti-terrorism design to be easier for security as well. Seating exists for waiting. At Moynihan they don't really announce what track your train will be on until like 5 or 10 minutes before boarding starts. So there's not really a way to know where to wait for 'your' train even if benches were next to the tracks.

Also, 30th St Station in philly has less than half the annual amtrak ridership as Moynihan, but roughly the same size building.