Kingsley Amis was very critical of Tony Powell's habit of including swathes of his life, thinly disguised, into his novel sequence A Dance to the Music of Time, saying "Doesn't he know that novelists are supposed to make stuff up". It must have been strange to have been a friend of Powell, and to know that what you said one year would be there in print the next, along with his commentary on what he thought and what he thought you thought; the same would apply to Galsworthy/Forsyte, Henry Williamson/Chronicles of Ancient Sunlight. It's odd that KA should have been quite so critical, since he is quite autobiographical himself. This is partly style: KA makes things sound like they are drawn from experience, inviting you to believe that they are non-fiction
Even if someone is willing to invent things, they will be interested in things that are close to them. This is partly why there are so many more accounts of the problems that a householder has with cleaning staff that accounts of cleaners' problems with householders. And hence, a lot of books about writers. It may not be pure laziness, though. Writers know about words, and care about words (or at least they ought to). They will therefore wish to have their characters say things or think things using a rich vocabulary; they will also want them to say things about other books and poems. And here comes the issue with credibility.
I started reading a detective novel recently, with a title taken from T S Eliot. Oh, I thought, this writer reads! The detective was soon revealed as a matter-of-fact gruff Northern lad who had left school at 16 but who nevertheless spent his evenings reading Eliot and listening to classical music. Hmm, I thought- no reason why not, but... Then we met the next major character, who had a similar background; in her case, she had retired, and now spent her time reading French poetry and listening to opera. At this point, it lost me, since their cultural activities seemed to reflect the preferences of the writer rather than the characters. Because the reason they have been assigned these attributes is to allow the writer to comment and quote as he wishes.
The problem is that these days the canon has exploded. Rumpole of the Bailey may have existed as someone who stomped around court reciting the great 19th century poems, but what would his modern version recite? Auden? Eliot? Larkin? Lennon/McCartney? The canon used to have (or be given) a moral weight: you ought to know these, even if you don't. Nowadays people pick up what they want, and switch from Jane Austen to Discworld at will. This is good. But it's bad news for writers, because they can no longer assume that an echo of a Shakespeare sonnet, or a pun on a Dickens book title, will be understood by their readers.