Taxi drivers are usually contractors, so it's easier to get a job (sometimes as a sub) and its easy to avoid taxes. Think of it as a waiter job without the need to communicate well. Also, because of the off/hours nature, it's a good second job.

Because you have networks of contractors, you can lever social ties to get a job... This is especially important for immigrant communities.

Please spend an hour in traffic, and then an hour In your kitchen -- cooking for a hundred people -- and tell me what is less work.