There's a couple of problems with this:

1. People who work at starbucks work at starbucks because they don't have much money. And since they work at starbucks they *won't* have much money anytime soon. So they are completely priced out of starting a competitor (if their contract even allows them to).

2. Companies which try to pay their workers a fair wage have to compete with companies that abuse their workers and hire immigrants. Granted usually there's room in corporate profits to pay workers better, but if you are publicly traded you can't just pay your workers whatever you want because your investors get to tell you what to do.

2.5. Even if you do treat your workers right, manage to turn a profit and grow your business, you will be growing much slower and forced into a niche due to other companies having an economic advantage of using pseudo slave labor.

3. Usually people who are making shit wages don't have a lot of free time. They work long hours or multiple jobs and live paycheck to paycheck. Sometimes this is the result of overspending, a lot of times it isn't. This means they don't have much time to search for better jobs (and you do have to search for them as better jobs are rare to find since everyone wants one). They also don't have much time to learn new skills to branch out into different industries.