Why? I'm genuinely curious. Obviously total cargo volume is lower. Is there some regulatory concern that makes them "not comparable"?

The volume vs weight probably comes into play as well.

Without passengers, passenger aircraft probably run out of volume capacity long before they approach their max take off weights.

Whereas I think with dedicated cargo aircraft, it is usually the other way around - weight is the main limit, less volume.

I'm guessing they do a lot more pallet shipping, and your typical passenger jet isn't prepped to load / store / unload that versus luggage.