This is true.
People need to be able to take care of the plants, have a place to plant them, know what they are doing. In theory cucumbers and tomatos, for example, are pretty easy to take care of. They can also easily get sick and attract pests, they both need ample room to grow, and need a decent amount of water.

Depending on the actual plant and your location, it may cost more to buy a plant and try to grow it successfully than to just buy the actual vegetable.

In theory, it's a great idea. For a lot of people, it logistically isn't feasible.