Why is improvisation seen as difficult in music but so easy in speaking?
Suppose we could only speak by reading off a script; we would be very limited in our means of expression. If, furthermore, we never wrote a script ourselves, but only followed scripts written by others. To what extent could we really be said to understand language in such circumstances?
Yet that is how music is taught.
In vernacular traditions, at least, the player learns to play by ear, or to improvise right away. We know that improvisation used to be stronger in the classical tradition as well.