Strangely enough I can't find this subject on meta yet, as I heard it somewhere recently, perhaps a podcast? (I believe the one with Jon Skeet.)
Why do so many people waste their time answering obviously duplicate low-effort questions? As a recent example: Inheritance vs. interface in C# My first guess is they find it easy to gain rep with.
The problem is this really hurts StackExchange. Other more thought through questions get less and less attention. There are a lot more 'stupid' questions than people that can answer difficult ones. The combined effort of all the answers (even by +30k users) doesn't even come close to the effort put into the question by the OP.
I really believe answering low quality/duplicate questions should be discouraged. One way would be to discard all granted reputation once a question is closed as a duplicate or as low quality.