zlacker

You can grow some amount of produce, if you have a garden, but a lot of people don't have their own garden, and if they do it's quite small. To be entirely self-sufficient, you need quite a large area of land just to grow enough food for the entire year.

Most people don't have that, and can't afford that, hence why they take the route of earning money some other way, and using the money to buy food made by others, from supermarkets. They can supplement their diet with home-grown fruit and veg, but few can sustain their family on home-grown produce.