To answer your question about why "overlanders" generally don't use these cargo boxes, I can only tell you that I can see absolutely no utility for one.
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To put it in more detail, there's nothing that I could carry in a cargo box that I couldn't carry more easily, more safely and more securely inside my vehicle. The only possible exception I could think would possibly be fuel (because you wouldn't want the fumes inside the vehicle) but then you don't need a cargo box for a fuel can, you can just strap the can to the roof.
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I'll turn the question around and ask: What is it that you're carrying in the cargo box that you can't either carry in the vehicle or do without? I guess if you have a whole bunch of kids in the seats you could use that area for their clothing, but outside of that, I can't see a use for a box like that. I do carry stuff on the roof (camp stove, firewood, fuel) but anything I carry on the roof I don't mind exposing to the weather which eliminates the need for a bulky, expensive cargo box.
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To sum up, the cargo box is an answer to a question I never asked.
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EDITED TO ADD: I could see a use for it if you have specialized equipment that you want to keep segregated from the rest of the cargo. Maybe a fisherman's fly rods and tackle, or a cross-country skiers skis and boots, or even survey equipment - things that are large and bulky but not particularly heavy. But since I don't carry anything like that it would serve no purpose for me to have one.