Standards/regulations that the transformers almost certainly don't meet, produced in factories that don't follow any standards or regulations, and then add in the cost and delay in shipping something like that.
And yet somehow the biggest electric grid in the world is running on them just fine.
China has the most sophisticated grid in the world, and is spending $100B a year on expanding and upgrading. They have a uniquely high share generated by renewables. It runs 800kV and will go higher after the upgrade. The first Small Modular Reactor will come online this year. If you think that’s all just being built in random factories without standards you’re very much mistaken.
Maybe.
https://evernewtransformer.com/pt/how-to-purchase-power-tran...
If I were to desperately need a power transformer, I'd consider going down this route in parallel to waiting for years for a "blessed" one.