We're often talking about custom plants here, or plants made in very low volume. Making a simulator for that can be prohibitively expensive, since you can't amortize the cost. For particularly important/expensive plants it still sometimes gets done. Though note the simulator might not be able to capture all the nuances of the real world deployment, so it might not be sufficient.

Meanwhile having a separate copy of the plant is also not often viable, once the plants get to any kind of size.

edit: After having 'someone' fact check and remind me: there are quite a number of simulation tools available that can sometimes partially do the work.