Large transformers are built to a very specific custom specification. Each one is a custom engineered unit. A transformer is basically a sealed vessel full of volatile oil that you pass an extremely large voltage and current through. The machine has to be perfect. The smallest clearance issue, insulation problem, etc and you have a bomb. And that's just making it safe. We haven't even talked about making it effective.

Transformers are the sort of think that are not rocket science to build, but every single detail is very very important. It requires a team of people to design, construct, and validate and all these people need to have the skills from doing it many times before. These people and facilities simply don't exist in large enough numbers because they are very expensive.

There are also supply chain issues to consider - the steel used in transformer magnetic circuits for example is a very specific metallurgy and construction. During COVID it was quite difficult to find. Poor steel means poor efficiency which at the scale of these machines can mean millions of wasted mwh of power over time.