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.

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.

If you mean the middle rear, probably because there’s nothing in front of you. It may be the safest overall, not necessarily the safest in a side impact.

Those are both bucket seats, just to different degrees.

Imagine a camera placed where the spine or leg is looking at the side of the seat. Look at how much exposed surface area faces the camera. Let’s call that surface area the ‘side restraint component’. (IE, if the side panel comes ‘up’ out of the seat 2", and extends out 4", the side restraint component is 2").

On a seat belt, you’ve got about 2" x 4" surface area on each side. So 8 square inches on each side. That’s all a bench seat gives you.

On that car seat you’ve got about 2.5" x 8" on the back, plus an average of let’s call it 2.5" x 4" on the seat. So that’s about 30 square inches on each side.

On the racing seat you’ve got about 14" x 20", but cut in half as a triangle, and let’s say the shoulder bit fills in the missing part by the belt opening. So call that 140 square inches per side.

The car seat may be designed for comfort, but the side bolsters do have a restraint effect.

It’s one of the benefits of a bucket seat, and you’ll note front seats have a bucket shape both on the back and the bottom. This does a LOT to keep a human in place, especially if the seatbelt is holding the human down into the bucket. Lots of surface area on the side of the leg and torso for the bucket shape. OTOH with a bench seat there’s nothing at all keeping the human in place, there’s just the 3 places where the strap crosses the human and those don’t do very much. Seat belts are designed to keep you down in the seat.

This exactly.

Bench seats are great for hanging out with afore mentioned dog and/or girl.
Bench seats are super shitty in any sort of side impact collision. There’s nothing keeping you in place except your safety belt.