Here's more or less the same load, but in 2018. There was also ample seating space for four youths, in the CrewMax configuration of the Sienna's sibling, the Toyota Tundra.
Yes, it's true that good weather is a prerequisite for throwing everybody's stuff into the bed. But as you can see, a tonneau cover would have kept rain out, as would a cap from an aftermarket outfit like Leer.
Even a simple tarp with some tie-down would have been OK in my situation.
You know how long it took to load this much gear? Less than five minutes.
The other factor is that while smaller pickups can also accommodate a lot of stuff, full-size pickups often have a more plush ride. That was certainly true of the Tundra, which cruised in near-total comfort for 200 miles.