CEDICT
Wiktionary
為什麼不。反問語氣,表示應該、可以。
重編國語辭典
為什麼不;表示反問語氣的副詞,暗示提問者認為應該、可以做某事
維基詞典
Why don't we go home?
Source: Tatoeba
Why don't you give her some flowers?
Rather than cutting down on cigarettes, why don't you just give them up?