I don't get why more people don't do this.
The answer to that question is simple: gardening can be a chore. The benefits have to outweigh the effort, and I think for a lot of people the effort is too much. I used to keep a garden, but I found that I did not get that much enjoyment out of tending one. It was great to have the fresh tomatoes, eggplant and chiles, but not THAT great. Even though it wasn't for me, I think you will see more people starting to do this as the effort/benefits ratio begins to tilt. If I ever do it again, I think I'll try just putting out a few plants in pots, rather than lots of plants in beds.