Longtail cargo bikes - why no internal-gear hubs?
I suspect the main reason is cost - internally geared hubs are more expensive than derailleur setups.

Torque capacity may also be a factor. You are wrong about the force the hub has to withstand being only dependant on the rider. The torque on the hub is determined by the gearing ratio and crank arm length as well as the force the rider can apply.

A heavy bike will require a lower gear ratio between the chainring and sprocket to lower the overall gear ratio range. A lower gear ratio means more torque on the hub input for a given force applied to the pedals.