Because 7 dollars isn't livable.
The minimum wage is supposed to keep people who full time work above the poverty line.
Let the market set the wages. The market is more efficient. If someone can't live off of burger flipping or toilet scrubbing, let them get two jobs. Or three. Or two jobs and the other time for votech or community college education-both already largely subsidized by the government.
At one point in my 20s I worked weekdays at a full time construction job and evenings at a fine dining restaurant. Weekend mornings I did valet parking at a ritzy hotel. It didn't kill me. But I never stopped! The situation helped me realize I needed to make a change. An older semi retired fellow I worked with told me "Burn, what the F&*^ are you doing? Go to school!" I did.
Another paradox of a high minimum wage is that high school kids are disincentivized from going to college. It is counterproductive. If menial labor pays good money, the competition for those jobs increases. If it does not, people work to improve themselves and move on to better things. With a dearth of applicants for a job with shit pay, the wage rises automatically to its natural level.