Actually very little bulk grain is shipped in containers as it is much more efficient "in bulk". Containers are utilized for what used to be regarded as general cargo and can contain pretty much anything up to about 20 tons weight. One of the problems with Traveller trade for me (as a reallife 7 term Merchant) has always been the small sizes of the cargoes being shipped that then take a week to discharge and reload. Typically IRL we could load 200,000 tons of bulk in two to three days max. A few hundred tons woul not take much more than an hour at any decent star port.
Just my 2 Cr
You are absolutely right. OTOH, where's the adventure and chance for riches transporting 100,000 cubic meters of sourghum? PC adventurers don't have the kind of ship needed for that kind of cargo and can't compete with the bigs anyway.
I suspect the cargoes you have handled do exist, it just doesn't apply to the level of player characters to consider them.