I'm going to lean on Merriam-Webster for this, even if Obama might not approve of dictionaries.
* Main Entry: free·lance
* Pronunciation: \ˈfrē-ˌlan(t)s\
* Function: noun
* Date: 1820
1. a: usually free lance : a mercenary soldier especially of the Middle Ages : condottiere b: a person who acts independently without being affiliated with or authorized by an organization
2. a person who pursues a profession without a long-term commitment to any one employer
The term "freelancing" includes the component of working for someone else.
In any situation where you are selling your own professional services, like a doctor or a lawyer, you will be limited by the number of hours in a day. The problem with freelancing is scalability.
Doctors and lawyers can become moderately wealthy, but most of them do this from investing their earnings.
To become truly "rich", you really have to work for yourself -- and get other people to work for you.
Very few top notch SEO accept client work. Instead, they have discovered that it is far more profitable to work on their own sites. In addition, it's a lot less hassle.
Freelancing is a great way to get started and it's very useful in funding your long-term projects. But, I would counsel against freelancing as a long-term strategic plan.