• Sometimes you need to move a thing that is oddly shaped and doesn't fit within the confines of an enclosure

Like what? And is that a common use case?

• Depending on what you're hauling, you may want separation between the cab and the payload. Like if I'm moving dirt, I'd rather not have it rolling around my cabin

Or just put down a nylon sheet, put the dirt on top, fold the nylon sheet over it and bind it down. Now it is covered under and over and will not fly around.

In most cases I guess people will just buy prepackaged earth in bags. That also doesn't fly around.

Sure, if you are one of the very few people that work in the woods or on a field, where this common use case, then alright. But that would not explain why those cars are so common.

• Easier to clean, just take a hose to it without needing to worry about soaking the cabin

Buy a bus with removable carpet, then you can just hose it down as well. Many buses have a small step, which separates the cabin from the back, so water will not flow into the cabin.

• Access isn't limited to just the door, which can be useful when unloading something

There are many different rear door types and sliding side doors on the side that provide ample and easy access. This isn't difficult or complicated.

That didn't convince me that pickup trucks are not a very specialized vehicle for just some uses, while transporters and mini busses are much more useful for all kinds of purposes. Be it furniture, tools, sport equipment, electronics and other sensitive equipment, and people. While also being good at hauling the occasional dirty stuff, if you just put something underneath.