Which of course isn't true. If it were true, skill-based games would be just like levelling games: the more you played the better you'd be. But they're not. In part that's due to natural differences in talent, and to skills learned in prior games that apply to the current ones. (Even levelling games have a certain amount of skill involved, with better players progressing faster and capable of more difficult challenges than inferior players of the same character level).
But a lot of it is that skills are acquired not merely through performance, but through the conscious effort to learn the skill. For myself, I find that simply doing something for a while will make me better at it until I reach a plateau. After that plateau, doing more of the same doesn't make me any better.
And at some level, I think that performance stifles my ability to improve. I once wrote, in full knowledge of the irony: "I'm too busy writing a novel to learn how to write a novel!"
I did learn some things about writing a book by writing a book. But there's other stuff that I still haven't figured out, like "how to write a book concisely" or "how to write action" or "how to build tension" or "how to work in reversals of fortune to keep the narrative interesting". I'm not sure I'll ever learn those things just by writing.
More subtly, when I'm focused on producing, I'm not focused on improving. If I read a book for enjoyment, I don't notice most of what the author does well or badly, or how she does it. I only see the product and not the process, not the pieces. To notice those things I need to pay careful attention, to keep in mind what I'm trying to learn and see how it's done. When I'm trying hard to make the writing happen at all, it's more difficult to think about how I might do it better.
With this in mind, and also because my muse quit a few weeks ago and I haven't been able to woo him back or hire a new one (does anyone know where to get a good muse?), I dug up my copy of Creativty Rules!, a book of writing exercises that I started a few years ago and only got through a few chapters of. Maybe now that I'm not writing anything, I can figure out how to get better at it.