I do not think that this is a valid comparison.
It would be more like, why don't farmers produce their own food (and rely on packaged food instead).
The modern world is built on specialization: most farmers have specialized in some direction or another, and the ones who haven't usually make less money. At best, a farmer with livestock will occasionally take a pig or cow off to be butchered (since butchering is another specialty entirely, needing specialized equipment to do well), or a farmer growing corn will plant a few rows of sweet corn in a field near his house, or a farmer growing produce -- usually just a half dozen varieties -- will keep back a little for himself.
But the calories a farmer will produce for himself are generally a small part of his intake. In the end, it's a better use of his time to focus on producing as much of a handful of crops as possible and buy what he needs from a grocery like the rest of us.
The same argument applies to almost any category of product, with varying ramifications unique to each.
If I made my own soda, I'd leave out high fructose corn syrup. Why do I let Coca Cola poison me? And so on and so forth.