Why? With chat history present, reasonable people can search for previously answered questions. Without it, if somebody joins afresh, even if their question was answered just before them, they could have no idea.
IMO it's partly because it's an inefficient platform for storing knowledge, its purpose is instant communication.
If that knowledge is actually valuable, it should be stored elsewhere, in a structured manner. But I get your point, it's better than not having any answer at all.
