I never write about characters in isolation. And I can't think about a novel that is about a character in isolation. It would be a very experimental novel. Very much a break with what we consider the novel to be doing. Generally novels are about situating characters within social settings, and obviously within initmate relationships, personal relationships and families as well. That's how I conceive of character. A character to me is someone who is engaged in one or more dynamics at a given time, and I can't actually imagine what it would be like to write about a character who was just on their own all the time.
Out-of-context commentary from Sally Rooney, author of Normal People, during a literary event at London Review Bookshop
This statement is probably true for most authors. What kind of reader wants a story about a character who's on their own all the time? It would be like reading a novel about me! Although I find myself quite entertaining, an author would probably prefer to focus more on my cat and his enemies.
That said, it is a little sad that it takes so much imagination to write about solitary characters. The not-so-normal ones.