Why does cabbage grow above ground
but carrots grow below ground?

Of course, you know that both have parts both places. Almost all garden plants have the same kinds of parts. They have roots and stems and leaves. And most of them make seeds. Some kinds of plants, like lettuce and cabbage, make leaves that are good to eat. Some, like carrots and turnips, make big fleshy roots that are good. And some, like wheat and corn and rice, make seeds that we eat.

Out of all the very many kinds of plants on earth we have picked a few to grow because they have a part that is good to eat. There is no law that says you can’t eat a corn root or a carrot leaf if you want to. But these are not likely to be very good food, and I doubt that you will ever find them in supermarkets.