It's said in the article, I think it's because the exact boundaries of operation depend so strongly on local conditions the specification space has too many parameters in variable space: easier to make them for the situation to spec than build to a standard?
Down at the domestic supply level I am sure they buy thousands of mostly identical units.
Sure, but how much of that is strictly required? It makes sense that transformers have traditionally been completely custom designs, but does something like cable orientation really matter all that much when you've got dozens of projects being delayed by many months? Why not modify the substation design to fit the transformers which are available?
Down at the domestic supply level I am sure they buy thousands of mostly identical units.