Because they are not going to miss out on holiday sales. See the late 2016 MB as a prime example. Kaby Lake wasn't ready. They could have waited a couple of months and simply released the early 2017 model.
If these rumors are true and all the others why not simply delay it to January or something. No point rushing an artificial deadline to release mediocre product, and we are all doing fine with the current phone. If the iPhone 8 comes out in tiny numbers and missing a lot of features I'm gonna wait anyway.