Have you ever looked closely at a mail truck? (If you’re in the UK, substitute a milk float.)
It’s a crude design... The bolts are exposed. The gas cap is visible. The corners are square. The windshield is nearly upright.
But so what? Most of the time, it travels no more than 30 mph. Aerodynamics — no need. Style — why should they bother? Speed — next to none. Cushy ride — well, the postal worker might appreciate better.
The cheapest car you can buy has a much smoother design. Its interior is way more refined. But its intended function is different. And car manufacturers have hugely more competition for than mail truck creators.
Takeaway: Don’t put too much effort into something that doesn’t need it.
Another advantage to the US mail truck design is that they discourage theft. If you stole a mail truck, it would be obvious that you did so unless you did a huge amount of body work.
I heard this pointed out in a talk on the One Laptop Per Child project. The laptop is designed so that it would be obvious where it came from if an adult stole one.
Interesting fact for you (not about the van, but about the postie): in Sydney the postman delivers the mail either by motorbike or bicycle.