We learn a new skill just enough to become competent.

Take driving, for example. At first, you'll be nervous, but with deliberate practice guided by a tutor, you'll get better. When you reach a base level of competence and pass a test, you'll get your license and be on the road. 

But for years, you'd find you haven't really gotten better at driving. You only reached a base level of competence that will get you by and that's it. 

The same goes for writing. School teaches us how to write until we reach a certain level of competence. But we don't get better by practicing what we already know. And that's because we are hard-wired to avoid pain.