I spent two weeks in the hospital from my chronic insomnia once, anybody who tells me I just need to relax or stop reading in bed is about as helpful as someone who tells somebody with chronic depression to cheer up.
Not everything is in your head, and no amount of positive thinking will change that.
I think CBT is training your mind to work together with you. Some of the most important things in getting your mind to cooperate (to achieve a result like better sleep), is A) believing that you can, B) Wanting it, and C) Putting in mental effort to achieve your goal. No, it's not a panacea and not guaranteed, but the science* is clearly there.
Not everything is in your head, and no amount of positive thinking will change that.