Because a thing which you believe and yet is not defined by the scientific method (or rather, empiricism) is known as "faith", a concept which you and the rest of this thread has spent a good deal of energy attacking.
In other words, it has no empirical base. It's an idea. A thought, held in the belief that some future action by you or others may prove it correct.
That variable certainly exists - it's why we have blind-controlled trials as the gold standard of research.
As compared to a hypothesis - here is an idea that should be tested by investigation, I make no prior judgement to its truth
I don't think a true null hypothesis exists as long as humans are involved in concocting them, the person could want, consciously or subconsciously, any output from any experiment.
Maybe one outcome leads to more research that's a major paint to secure funding for and one is much easier? Maybe the outfit funding the study clearly wants one particular result?
Which goes right back to what I'm saying: humans are not purely logical, true null hypotheses don't exist, and the only difference between a "hypothesis" and a "belief" by what you just described is the degree to which the person with the idea wants a specific outcome - a variable which is completely unrelated to the eventual truthiness or falsity of the output.
As such it doesn't matter where it comes from so long as it is tested before acceptance. Which is why I struggle to understand the bootstrapping problem that the other poster talks about.
--edit-- To put it as simply as I can muster - why does the origin of the idea to test matter, when it is the testing that's the important part?
But not all things can be tested - think about dual slit experiment. You can't reproduce the experiment in the sense that you'll know which slit will be taken.
The point I was trying to make is that the origin of a hypothesis is not really important, just that before it is accepted as true or false it is tested.
--edit-- this wasn't actually an edit so much as a reply. Oops!
Quoting Hamming: "A man was fishing with a net in the sea. He concluded that there is no smaller fish in the sea than what he caught". (s/fishing net/scientific method)
(1)Empiricism modulo quantum effects - and here we go again.
Scientific method likely cannot resolve all possible categories of question, no. Empiricism is a fundamental assumption, that things are repeatable and hold true under investigation.
However this is a very different sort of assumption to the rejection of evidence based on faith that is implicit to creationism. One is our best effort to understand the world around us, the other is wilful ignorance.