Fragmentation of knowledge. The SO model tries to get the answers in one place so the people who search first find the answer in one place.

If duos were open, the you'd have to look in all the places first to find the best answer. And then you might as well skip that step and ask another question rather than search.

Try searching a forum or /r/javahelp or the like for an answer. See how many results you have to hit before you find an answer that works for you. Then reconsider the value of closing and pointing people at the duplicate question.