I think the answer is because Physics.overflow should put physics.stackexchange out of business regarding grad level questions and higher. They are not competent to answer these questions, due to their broken political policies, they censor anyone knowledgable with their blocks and their power-mad administrators. We will never do that, under any circumstances, since we trust the community.
While copying is not a good solution in the long run, we can extend the answers here, and gradually improve the quality of the answers using a better community. Replacing the broken nonsense at stackexchange is one of the goals in setting up the site, but of course, we can go much much further than stackexchange intends, because we are interested in academic discourse, not in commercial discourse.