The correct response to Hamming's question is "Because I have a comparative advantage in working on the problem I am working on". There are many, many important problems in the world of greater and lesser degrees of importance. There are many, many people working on them, even within one field. It does not make sense for everyone to attack the same most important problem, if indeed such a single problem could even be identified. There is a point of diminishing returns. 100 chemists working on the most important problem in chemistry are not going to advance chemistry as much as 10 chemists working on the most important problem, and 90 working on a variety of other, lesser problems.