What does "why not?" mean in English for Spanish?

In English for Spanish, "why not?" means:

¿por qué no?

Listen to "why not?": (If you have an HTML5 enabled browser, you can listen to the native audio below)

This is a phrase that is used in the GamesForLanguage English for Spanish Language Game in the following scenes:

• English for Spanish 4, Level 2, Scene 5