If they are to be interchangeable while also being paralleled they have to have very nearly exactly the same reactance or they won't share power evenly and there will be circulating currents between the transformers wasting energy.

Keeping a stock of million dollar items in case one fails once in fifty years is pretty poor use of capital. By the time you get to use it the standards will have changed. And how many different transformers will you keep in stock> You can't reasonably use 500 MVA transformers everywhere, some places only need a 250 MVA unit and might not have space for anything larger. Which voltages will you choose and what will you do with your 500 kV transformer when the backbone gets upgraded to 650 kV or 750 kV or 1 MV?

Do you think that the people who run electricity distribution systems don't think of these things?