Why publish the method instead of keeping it private?

Because a method that cannot be argued with is not a method, it is a brochure.

There is also a self-interested reason worth stating plainly: a claim first made on the day a product launches is worth very little, and the same claim made before there was anything to sell, still standing years later with the misses left in, is a different kind of evidence. Publishing early is how the record starts.

The constraint that comes with it is not opening a through line the work cannot sustain. Everything on this site is written with the assumption that somebody will check it later.