actually that little extra material makes a big difference.
Look at it from the side (IE put a camera where your spine would be), same on the bottom where your leg would be. There’s several good square inches of ‘wall’, much more than just a seat belt.
And while it is angled up somewhat, the seat belt is doing a great job pulling you back down into it.




Ever look at the back seat of a modern vehicle? It’s almost never a flat bench. There’s indentations, they’re just less deep than the front seat.