Work at a dishwasher factory. We used to make a model with windows, they were really expensive parts, which meant that they were really expensive dishwashers for a feature that really isn't useful.
It makes sense in a microwave or oven because you can check in and make sure it's all good, or pull it out if it's done. You can't do that with a dishwasher, it just runs it's course.
Plus all you could see in the thing was splashing soap water.