Because it is a question that can't provide a positive signal, only a negative one, and since the negative signal is kind of flaky. I.e. somebody "passing" this

Because it is a question that can't provide a positive signal, only a negative one, and since the negative signal is kind of flaky. I.e. somebody "passing" this by writing some code doesn't actually give you any confidence in hiring them. It is only going to filter out people who can't code at all, and then you have to ask them a real question. But due to giving the candidates no guidance, some who actually know how to program will not just bang out something simple in as minute. They will think it is a trick question; or waste time choosing what to write rather than just do it; or start writing something far too complex for the purpose of just passing the quick filter. It is a profoundly shit question, setting up candidates for randomly failing.