From Longman Dictionary of Contemporary Englishwhy not?why not?REGRET/FEEL SORRYused to say that you agree with a suggestion ‘We could invite John and Barbara.’ ‘Yes, why not?’ → why
Examples from the Corpus
why not?• "Maybe we could drive over to the beach today." "Yeah, why not."• But if every now and then real life is generous enough to throw us a dizzying love affair, why not?• But then ask: why not?• Sure, and why not a cane to poke at baskets?• Demand immediate repeat immediate repeat immediate dispatch of krytron or immediate repeat immediate repeat immediate explanation of why not available.• Since so much of house is pure beats, why not buy your funk in skeletal form too?• If you have a small bathroom, why not consider a radiator with a built-in towel rail?• But why not reward teams that played the best during the entire year?• Well then, why not simply erase them? 