While free work is great, and can quickly advance your career, there's another part to this equation:

The free work can and should be done virtually (a.k.a. remotely), so you can work with people who are in other parts of the country / world.

There are many reasons for this. The most important reason is that it removes nearly all of the risk for the potential employer.

Employers worry about wasting their time and money whenever they hire someone. With free work, you obviously remove the risk of money, but with virtual free work, you remove the risk of wasting their time.

If you're not in the same office as them, they don't have to spend a lot of time training you -- they don't even really have to think about you.