One of the downsides to using a van for these heavy duty use cases (speaking from experience) is that they're typically not equipped with powerful enough engines. So you end up straining the engine when towing heavy loads, which reduces the life of it considerably.

Also for a lot of vehicles, a GVM/GCM upgrade is needed to be able to tow certain loads.

Not defending these large american trucks. I think there are valid use cases for them (in smaller bodies), but the majority of the ones I see driving around are just for peoples pleasure and not utility.