How much time and effort do you put into growing your own food? Can you support yourself if things were to go bad in the long term? How much of your food planning revolves around having a quantity of a food staple that you can't readily grow?
The problems are several... One is economies of scale - it's much more efficient to grow a lot of one thing, hence having mega-farms. Different plants have different needs, and if you're growing to sell, trade, or preserve, you generally want as much available in one go; when you're growing for immediate eating, you only need moderate amounts. It is easier to process (eg preserve) a lot in one go, rather than the time, effort, and fuel needed to do lots of small amounts.
But - you often need more than just one thing.
I raise this thought often when I make jam. Usually a few jars at a time, from fruit and berries that I grow. The simplest (and often best) recipes are simple - the fruit, a large quantity of sugar, and the juice of a lemon. The lemon provides pectin - the setting agent. The fruit is obviously needed. It's the sugar which could be the problem - I do not have the space to grow enough sugar, be it in cane or beet form, to deal with what I use. Easy enough to solve in the short term - it keeps for many years if kept dry. What if it was more than a short term problem?
Does the space required for growing sugar balance the space needed for wheat/corn/barley/etc.? Who around here would grow grain, and trade for what I choose to grow.
Some problems would be solved with having more space to plant & grow - but that would also mean needing more help to harvest, weed, maintain... This is one of the reasons that scales of economy work - it's easier to grow more of a few things that some of a lot of things...