Because as with most car designs back in the day safety was a secondary consideration (if at all) to style. Firstly because these types of seats provide absolutely no stability so everyone goes sliding all over the place the moment anyone goes around the corner at speed, and secondly because there are no actual independent seats it encourages everyone to bunch up, even if there’s only actually enough seat belts for two people.

These problems are mitigated if you slow down although they’re still there.