Unfortunately, one of the insulating fluids used in the big transformers (including the ones up on the power pole) was PCB. Nasty stuff. Its use was banned by the Feds around '79-'80. Not sure what they use these days but I suspect whatever it is is probably not worth the effort or expense to jump through the EPA hoops. Hence the "Let somebody else make them" attitude. We do still have a few manufacturers of big transformers but as you stated they don't keep them on the shelf in stock. Build as needed.
Almost all manufaturers use plain old mineral oil with additives for transformer coolant.
The real issue is all the old transformers being replaced by “new” transformers. It isn’t very easy to swap a broken/burned out one for a new one.
And that’s if you can get one made specifically for your generator. We’re talking thousands for just the USA. Plus the controls and safeties for the same. -—I can guarantee you it’s not just the the transformers. It’s much more than that