What is there against it? / I don't see any reason why not. / What's the sense of not doing that? / Why not go ahead with it? / Why should we not be happy to do it?
lit. "Why don't they eat meat?" (said by Emperor Hui of Jin 晉惠帝|晋惠帝[Jin4 Hui4 di4] when told that his people didn't have enough rice to eat) / fig. (of people from higher class etc) to be oblivious to other people's plight