It certainly sounds reasonable to have an enclosed cargo area with sufficient head room for comfort, but saying they look like they do because there are requirements doesn't really explain it.
My rural carrier had no problem putting mail in my mailbox from a modified jeep. They would stop and get out for packages of any size (putting them by the house).
In town here, it's all walking routes. I wonder what the numbers are on curbside vs walking?
Modified jeeps are the best you can get without the leverage of being able to write 9 digit contracts. That doesn't make them ideal.
Aftermarket RHD modified vehicles have a lot of issues, like being able to see the gauges, the ability to operate all of the ancillary controls, interference with factory safety equipment, poor ergonomics, etc.
And those jeeps had generally terrible visibility. You're making a lot of statements with little to no data, nor even research on to why they would want to improve.
I made 1 statement saying that requirements aren't explanations, 1 statement that I observed a rural carrier use a jeep for years, and then a statement that jeeps are in fact commercially available.
I would certainly expect a jeep with a mirror bolted on the traffic side to have better rear and side visibility than something with no glass in the back, so go ahead and hang your hat on visibility if you want.
It certainly sounds reasonable to have an enclosed cargo area with sufficient head room for comfort, but saying they look like they do because there are requirements doesn't really explain it.