Sure, free jazz and 20th century composers played a key role in this. But musicians always improvised, although with much tighter rules. Beethoven is said to have been able to improvise whole sonatas.
I'm just messing with you, I enjoy free improvisation to a degree. Personally, I get more out of listening to someone find their way through a set of changes, in time, because it provides a little more context, but I dig this kind of thing as well.
You are right about composers being able to improvise though, it's not as if writing all of that music was really a laborious task for say, Bach.