It's useful to see the others, not sure how useful it would be to see inside a dishwasher. Could be fun though. Also probably is cheaper not to have it and could be better insulation.

Same reason fridges probably don't have windows even though that'd actually be handy. I once saw someone post about how their really expensive fridge actually has a window, but their mom put a curtain in front of it because seeing inside the fridge looked "messy". What a travesty