Definition of "why not" []

• Why is that not true? (adverb)
• Used to state that one has no objection to and approves of a proposal. (adverb)
• Used to introduce a proposal. (adverb)

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• "Since at any moment a cell is organizing amino acids at the incredible rate of thousands per second, why not store the information in the nucleus directly as a string of amino acids, or have twenty different baselike molecules, each one indicating a specific amino acid, rather than the three bases that nature actually uses?"
• "If they teach warfare at the Graduate School for Political Management, why not teach political management in the military?"
• "There are other multilateral agencies covering health (the World Health Organisation), development (the World Bank) and happiness (the whole of the UN), all of which help women as well as men, so why not just leave it at that?"