CEDICT
Wiktionary
为什麽不。反问语气,表示应该、可以。
重編國語辭典
为什麽不;表示反问语气的副词,暗示提问者认为应该、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?