why not后加什么?

If you love each other, why not get married?

要是你们彼此相爱，干吗不结婚呢？

Why not have the dish of the day?

为什么不吃今天的精选菜？

Why not write to her?

为什么不给她写信呢？

Why not tell her you're quitting and have done with it?

为什么不告诉她你打算辞职，尽快把这件事了结了呢？

I didn't want a part in the play, then I thought ─ stuff it ─ why not?

我本来没想在剧中扮演角色，然后又一想——管它呢——干吗不演？