Why is the average dayrate so high compared to the average salary?

I see loads of dayrates between 500 and 700. Also seeing people say that if well optimized a €600 dayrate results in a €6000+ net salary? If you compare this to normal monthly wages you come across in r/BESalary I wonder how the difference is so huge? I also wonder if there is a big freelance market besides the IT sector like the engineering or financial sector?