Everyone thinks they can write. Depends on the meaning put to the word "write." Any literate person can put two words together or explain a problem, expound on a solution, or describe something. But that is not what's usually meant. What people who speak about writing mean is this: writer == author.
Much more difficult to do. Because author == I F Stone, Steven King, George RR Martine, etc. They are not referring to a memoirist or an historian, both occasionally very hard jobs.
So we've narrowed the discussion. writer == fictionalist. Everyone thinks they can do it. Everyone can hold a wrench, too. That's the least part of being a master mechanic.
Part of the problem with this societal hallucination, it's fed by writers themselves. Look over any reference section in a bookstore and dozens - if not hundreds in a well-stocked store - of books on writing pop out of the shelves. I've bought them. You have, too. Be honest, you have.
What have I learned from these books? What does anyone learn from these books, besides certain basics involving language use and a few hints on plot and characterization? How to write like the author who wrote the book. Read Steven King's book on writing and learn to write JUST LIKE KING. Read Bradbury's book on writing, a nice one it is, too, but once again, it will teach you how to be Bradbury. Syd Field's books on screenwriting...how to be Syd Field. Buy Save the Cat, learn to be Blake Snyder.
A novice can pick up meaningful advice in these books. They aren't useless, only of limited worth. How does one become a creative writer, a fictionalist? How does one become a good computer programmer? Through study and practice. That's it. Study and practice. No secret handshakes, no deals with the devil, and no unique knowledge. Study fiction, read and read and read. Work at reading. Write, and then write some more, then write even more. But work isn't enough. Banging away at a range all day ain't gonna turn you into Carlos Hathcock. Hathcock was trained to shoot and embraced his training - internalizing it. The writer must do the same. Self-criticism is the best tool a writer has. When combined with study and work, it is an unbeatable combination. Being well-read, the good writer will intuitively compare his work to the masters, judge it, and make corrections during the writing process.
Lastly, for today anyway, the good writer realizes he is human, prone to error. No one can be perfect. Try to correct one's faults, but be prepared for the occasional failure. Remember, Shakespeare wrote, rewrote, and rewrote again as each of his plays was staged. He saw what worked and what didn't, accepted the failure and corrected it.
Not that I know anything about anything, just my opinion on the matter.