Modern cars bend and flex during a crash, and they do it in such a way to keep occupants safer. Bench seats can't do that as well. They also don't work as well with modern air bags and seatbelts, and they often lack headrests.
Without a headrest a relatively low speed impact basically snaps your neck and whips your head into the dashboard.

You want your seat to basically hug you and lock you into place. There's a reason racecar seats look like they do.