Because most people really don't want or need to see what's in there.
If you want to, open the door during the cycle. It won't harm anything.
If you're curious to see a dishwasher in action, some appliance showrooms have a display model, where the spray parts and the racks are in an entirely transparent box. They're to show how powerfully and completely the stuff inside will get cleaned, but you can also see how the dishwasher is intended to work, by filling with just a couple of inches of water, and then recirculating it for the duration of the cycle, to loosen the stuff on the dishes.
Answer:
1. Some dishwashers do have windows, but they're pretty rare.
2. Looking at a bunch of soapy, dirty dishes isn't visually appealing.
