In a washing machine the objects that are being washed move around. It's a dynamic process, that is unpredictable and always unique. Watching the movement is fun, it indeed gives a sense of control to the user, even if it is not actually required. Quite a few people even find it relaxing to watch the washing machine.
The window also prevents users from trying to open the door when the machine is filled with water.
In addition, it might be useful to some people to actually see what is being washed. If I come home, and see that the washing machine is packed with white fabric, I know I can wear my favorite white shirt soon.
In a dishwasher the items don't move. Making a window in the dishwasher would not expose anything interesting to watch at all. Given the working principle of a dishwasher (same water being reused as much as possible), watching his internal process might actually give you a bad appetite and convince you from hand-washing the dishes.
A dish washer also does not fill the whole machine with water. It actually uses very little water. Opening the door while this machine is filled with water would not result in water pouring out of the machine.
Another argument not to have a windows in the dish washer: a benefit of having a dish washer is that you can just put any dirty dish, cup or whatever straight into the machine, thus making the kitchen look more tidy. If you would have a window in the dishwasher you would be looking at dirty dishes again for 90% of the time.
On the other hand, dish washers with a window DO exist.
Electrolux brought this machine to the market after "market research", but it appears to have been removed from their current product range.