Emin misin? Neden olmasın?
- Are you sure? Why not?
Mümkünse, neden olmasın?
- If it's possible, why not?
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?.
... Well, why not? ...
... Why not? ...