It's just that in many circumstances, you can't change the voltage just because you create the standard.

Changing the standard for voltages used for trains for example would require huge investments. So you get standards that just documents existing rail road practices (required for validation of new products intended for existing rail roads) and you get new standards for the creation of new lines using new infrastructure and vehicles.

Many industries have investments in millions or billions of dollars, in which case they will continue with whatever voltages they already have until they are ready to build completely new plants. The plant just has a custom transformer producing the required voltage.

Same thing at the power plants. There are generations producing very different voltages. But the power plant just uses a custom transformer station to get the standard voltage for the high-voltage power lines.

For really big motors, the market is very small. A large percentage are custom-built. So no problem if they have custom voltages.