Because measuring stuff (following a recipe) is easy. All you have to do is follow the instructions of someone else who's done it before.
But what if you are making something no one has made?
What if you are creating something that's never been created?
Then does what you measure matter? Maybe, but it's exponentially more important to figure out what you want to measure. That's what matters.
So maybe the better question is, how will you measure?