If I had to guess, price and availability. The grocery store route doesn't have the shelf life, but it is a fraction of the cost. If you normally keep, say, three weeks of non-refigerated food on hand (eat the stuff in the fridge on day 1, the freezer on day 2) and have rotation shelves, you are eating what you store. I can do that for one person for about the same cost as two cases of MREs which gives you about 12 days if you only eat two of them a day.