The big difference is that when you have a collision at a 4-way or 2-way stop, it is almost always a "T-Bone" collision, where all the energy hits at a right angle. These collisions kill, especially at speed.

A similar collision on a roundabout is almost always a "glancing" blow, at an angle, and rarely causes injury or death.

Roundabouts force you to slow down when approaching. Stop signs can be ignored. Roundabouts can't.