I often purchase ingredients for the next 2 or 3 batches at one time. Then inevitably, I change my mind and brew something else, use up some of the ingredients I intended to use for another recipe, my inventory gets all out of whack, and then it might take several more years before I finally end up brewing the 2 or 3 things that I'd originally intended. That, and/or, like Denny said, life gets in the way, the ingredients age, and then I wonder why I bought all that stuff 12 months ago, including liquid yeast which by that point is pretty dead, and wonder whether I should just buy one batch of ingredients at a time.
Then I buy 2 or 3 more recipes' worth of ingredients again, and the cycle continues.
Eventually I might learn. Maybe.