Why self-driving cars look like "toasters"?

Why do self-driving cars tend to look, well, like the Zoox you see in this post? They're optimizing for storage area and simplicity, obviously, but there is also psychology at work.

Still, cubes have another thing going for them: They're a little less scary, sometimes verging on the adorable. Their distinctly non-aerodynamic shape suggests calm and safety. That's a welcome visual touchstone for a technology that might freak people out. "You don't want something like a Cybertruck carrying passengers autonomously."

I haven't seen it pointed out that they look like tiny train cars. There is a subtle benefit in avoiding the suggestion it is a vehicle that is driven, such as forward-facing seats up front and dashboards. It makes it look like mass transit.