Actually they don't. Some contractors are able to negotiate higher hourly rates, just like some employees are able to negotiate higher salaries. Yet, just because some employees managed to get paid above average salaries that doesn't mean all employees earn above average salaries.
I've worked as a contractor for a firm. We billed out pretty high.
I also know some people who work for a certain company as "contractors" but just seems like a way for the company to avoid giving benefits. A shitty hourly wage and they "furlough" everybody for a few months out of the year so they don't really have to worry about actually firing anybody, just don't pick up their contract next year.
If you are a contractor, then you are running a business and you will have to do things that an employee wouldn't. One of those things is negotiating enough to cover the time between contracts. If you aren't good at negotiating then your business will not do as well.