I'd imagine that pinball is essentially 2D. Maybe some lanes and ramps would affect gravity, but I can imagine the few Z-direction adjustments aren't worth simulating everything in 3D.
When you already have accelerator fields (pretty likely for a pinball simulator) you can reuse them for ramps
The ball often jumps off the playfield in real pinball. It's not uncommon for it to jump high enough to hit the glass. You'll never get an accurate simulation if you simulate it in 2D.