Several reasons:
1) Tolkien’s life experiences were far broader and deeper than most people these days. He fought in World War I and was a professor of philology at Oxford.
2) He didn’t have any aspirations to make a living writing books. He obviously already had his job; The Hobbit and Lord of the Rings were things he did because he liked them. So he was free to spend decades discovering Middle-earth rather than being forced to meet a deadline because if you don’t publish a book every year or two, publishers get mad (as is the case today).
3) Most authors today simply don’t have Tolkien’s moral character or courage. This point probably derives point number 1.
4) Someone has to be the GOAT.
