Why is a self-employed professional called a “freelancer”?

The word freelance came out of the period between the fourteenth
and sixteenth centuries, when mercenary knights with no particular
allegiance would take their lances into battle for the prince or state
that paid them the most money. They were referred to as freelancers
by authors in the nineteenth century and operated much like the
gunfighters in the American West. Now, a freelancer is anyone who
works independently.