@rnv This:
No one thinks they can simply be inspired to write a song and, never having played before, just pick up a guitar and boom: a song. So why would writing be any different? Well, I believe it’s because we think we’re practicing all the time, by virtue of using language to, well, talk.
Reminds me very much of something the Australian poet A. D. Hope said at the beginning of an essay called "The Three Faces of Love," of which I unfortunately cannot find a copy online, or I would quote it for you. Basically, he suggests that what you say above is one reason why, in his time at least, no one gave much thought at all to the education of poets: since we are, all of us, experts (in that we speak it fluently) in our native language, there is no obvious reason why any of us shouldn't think we are therefore already “qualified" to be a poet. (That's my paraphrase and might be a little inaccurate, but I think it gets at the gist.)
Of course, now, with the proliferation of MFA programs, at least in the US, his claim no longer holds. There are an awful lot of people thinking an awful lot about how to educate poets (much less other kinds of writers), and this professionalization of what it means to be a poet has had all kinds of consequences, but that is for another conversation.