I think there are a few different reasons - It's one of those things that's less than obvious. You're new, you're here for help, and your *entirely on-topic* question gets closed as a duplicate - sometimes even before you get an answer.

Contrast this with many new users who post multiple identical answers rather than flagging to close. Duplicates are not something a new user will get easily. 

I've personally joked about how the duplicate suggestions in the title work better - so there's *less* of an excuse, but it's still something less than obvious to a new user. While search does suck, the most obvious dupes are obvious - and if it's the same problem with different wording, it helps searchability.

I've often chosen to suggest *differentiating* a question from the duplicate target. I guess its balancing the immediate payout of answer(s) and reputation  for folks who have asked and answered there, and the broader goal of building a knowledge base. 

There's no quick fix - literally people just have to get "this is how it works" or decide on a threshold of duplicity that as many people are happy with as possible.