There's a critical assumption in what you said that is false.
you will spend more time and effort and opportunity cost getting those seeds to ripen, than if you just purchased them at market
Many homegrown varieties of produce are not available in stores, because the commercial varieties are chosen for traits like durability and appearance, rather than flavor and nutrition. You are certainly welcome to maintain that those quality differences aren't worth the effort, but obviously many people think they are.
Personally, I tend to grow things that are expensive, perishable, or often out-of-stock, while relying on the division of labor to provide staple goods at low prices.
If you are actually able to grow something profitably, consider making a business of it. Congrats! You found something to specialize in! Your neighbors are better off buying from you than growing themselves.
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That's not at all the point. If I have a tomato plant on my deck and the store is out of tomatoes, then I still get to have some tomatoes on my sandwich.
Also, see the point about commercial vs garden produce traits. An activity can have very poor returns to scale without being unprofitable.
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If you value having redundancy enough, then yes, being self sufficient can be worthwhile. Most people would probably just forgo the tomato or substitute it until the market has resupplied. Likewise for valuing nutrition. It can make self sufficiency worthwhile, but most will seek nutrition from supplements rather than backyard gardens.
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You're obviously correct about what most people do, but most people are also pretty unhealthy and the point of advice is that people have room for improvement, so it's not really relevant what they currently do.
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