The Milanese Leonardo is easier to find if they are a writer
Paul Graham writes about the basis for great talent in painters being inevitably more then just genetic. Fifteenth century Florence produces dozens of exceptional painters, like Leonardo. If genetics was all it took to be a genius, Florence would still be produced great artists. Presumably there were other genetic Leonardos, as there's no reason to think the genetic ability was concentrated in Florence. So, what happened to the Milanese Leonardo?
This is the same as Thomas Gray's mute in glorious Miltons, who fill up the churchyards of rural England. Only, Milton was as much the product of his time as Leonardo. His work would have looked very different without Cambridge and the Civil War, but he was an isolated scholar who communed with books rather than people. Although we can easily create a list of writers who flourished in groups or pairs (Shakespeare, Swift and Pope, Wordsworth and Coleridge) they often produced some of their best work solo or not in keeping with the times. Shakespeare's sonnets were written when the theatres were closed. Swift wrote and re-wrote Gulliver's Travels on his own for four years. Wordsworth and Coleridge both produced excellent work after Lyrical Ballads.
But think of the list of writers who were never really part of as artistic group. Not just the Prousts and Miltons, but Helen DeWitt, Penelope Fitzgerald, Evelyn Waugh, Murakami, Robert Frost (who wrote a lot of his best work before and then after his time with the Georgian poets). It isn't necessarily the same thing for writers as for painters. The people they need to commune with to be Florentine Leonardos rather than Milanese Leonardos are other writers, but those writers might be long dead. Some writers read and re-read a single book over and over, as Larkin did with Christopher Isherwood. Others type out passages from novels to work out the techniques involved, like Joan Didion with Hemmingway. Frost was very reliant on two anthologies, the Oxford Book of English Verse and the Oxford Book of Victorian Verse. He met and lived with some of the authors from the Victorian book, but not until his first two books were written and ready to publish.
I suspect that Milanese Leonardos need other painters around them, and a culture to feed off, but that writers may not. David Foster Wallace, in his own words, lived in a cornfield. Billy Collins worked quietly on his own, waiting fifteen years between his first and second submission to Poetry, with no beatniks, no symbolists, no group at all. It may seem like Zadie Smith emerged from a Cambridge (and London) culture, but Cambridge undergraduates are pretty much never successful novelists. The only other person I can think of who produced a great literary book as an undergraduate there is William Empson.
Penelope Fitzgerald spent plenty of time in London watching opera and immersed in European culture, the basis of her last four great novels. But she also lived on a houseboat, in a Suffolk village, and worked as a teacher while she lived in a council flat. Those experiences were also worked into novels. Evelyn Waugh hated London and spent most of his life out of it after he got married. That didn't stop him writing Scoop. Frost worked on his farm.
Paul Graham's argument is about cities and is, of course, fundamentally sound. But his artistic analogy holds less strongly for writers. He says, 'I tried living in Florence when I was 25, thinking it would be an art center, but it turned out I was 450 years too late.' Of course the influence a city has is not always obvious and it's probably significant that Melville and Hawthawne lived in Manhatten at the same time but didn't meet. They didn't have to. Being in New York was enough. Could V.S. Naipaul have written at all without going to Oxford?
However, the fact that a writer like Montaigne can appear from nowhere, with nothing like Athens, London or New York to produce him is important. Darwin also lived remotely, hardly ever leaving his village. Writers are less bound by the constraint of cities, although they benefit from it. Living in the right place with the right people can be important, but plenty of great writers can work their magic without doing so.