Are truck drivers' salaries in any way related to the stuff they are hauling?

think “nuclear waste”.

My ex husband drove a tractor. To me, this didn’t make sense, he drove a chemical trailer for a while and a trailer hauling scrap metal. The scrap metal job paid much more than hauling chemicals.

My understanding is that the salaries are whatever is agreed upon by their union and management.

It has to do mostly with the financial status of the employer.
There are bonuses offered in many cases for loads delivered by a certain time. Some drivers work for a shipping company, so their load content can vary. Some companies have their own fleet.
I’m not sure what it is like for the indy owner/operators these days, or if anyone still does that.