They have terrible rear visibility because modern crash standards (and the laws of physics) require that. 80s cars had much better rear visibility, but the roofs would easily flatten and those cars were generally deathtraps for any kind of crash. Modern cars handle crashes much better with stronger roof pillars and a high "beltline" (the line between the metal parts of the doors and the windows) and side-impact protection beams in the doors, but that comes at the cost of visibility.

Luckily, modern technology solves this, and more, if you get the backup camera (now legally required in US) and the rear blind-spot-detection radar. Visibility with those is much better than it was in the old days, as long as you know how to use them. However, they do all come at a cost, especially the blind-spot radars.

And it's required by U.S. law.