It's a nice day, isn't it? Why not go out for a walk?
- Hace buen día, ¿no? ¿Por qué no damos un paseo?
Why not apologize and ask for his pardon?
- ¿Por qué no te disculpas y pides perdón?
Let's have pizza for lunch. — Yes, why not?.
I don't want to go out today. — Why not?.
Why not try turning the handle in the other direction to see if that opens the window?.