Just because it's so
I've been thinking about a thing. It is incredibly rare to find, in an 18th, 19th or even 20th-century novel, a disabled or ill character whose disability or illness does not, in the end, become relevant to the plot. Wilkie Collins has umpteen ill and disabled characters, far more than most, but to the best of my recollection, they are never disabled or ill just because they are: in the end their affliction will always be a plot point. The eponymous Miss Finch's blindness is central to the plot; Noel Vanstone's chronic heart condition is why he dies at an awkward moment in No Name; the mental backwardness of Mrs Wragge in that novel also enables the plot, as does that of Anne Catherick in The Woman in White. In The Moonstone, Rosanna's deformed shoulder identifies her in disguise; even Lucy's limp is there to explain her bitterness against men (embittered disabled people being a pernicious but common 19th-century novelist's trope). You'd think, in George Eliot's Adam Bede, that Mrs Poyser's delicate health was an exception, but not so: far into the book, we learn that it has been carefully seeded to give her a reason to be upstairs in bed at a time when she would surely otherwise have spotted Hetty's pregnancy, which goes unnoticed by the less sharp-eyed members of the household. And of course Silas Marner's catalepsy is crucial.
In fact the only exception I can think of in that period is RLS's Catriona. Quite early on in this novel, we learn that our heroine is short-sighted. The first time I read it, I was unconsciously waiting for this to become somehow "relevant". It never does, in terms of plot. Catriona is myopic, just as she is tall and grey-eyed; it's just part of who she is.
My unrealised expectation was, I think, prompted by Chekhov's remark about loaded guns (if you mention one in a story, somebody had better fire it eventually). There is sense in this, of course; one doesn't want to load stories with irrelevant facts. But to equate human disability with that gun is to assume a norm, and to suppose that any deviation from said norm needs an excuse connected with the plot. Relating to a different norm, I once asked a thriller writer (not a very good one in my view, but very well known) if he would ever include a gay character and he said firmly, no, because their sexuality would be irrelevant to the plot (as far as I recall, there wasn't a whole lot of racial diversity in his work either, presumably for the same reason).
But a writer sets a story somewhere in the world, and if his world is populated exclusively by cis white physically and mentally perfect specimens, it had better be a fantasy world, because it sure as hell isn't this one. There is a reason to include characters who don't fit supposed norms, not for the plot but simply to make the world of the book credible.
One would suppose, indeed hope, that contemporary novels would recognise this, and maybe they do; there are so many, and I probably haven't read enough to know for sure. (Though as late as 1980, the blindness of Jorge in The Name of the Rose is still crucial to the plot.) Is it more common, in the 21st century, for writers to include characters who are ill or disabled just because they are?