Yes, this, and what @JaninaDuszejko said. (I write novels.) Yes, in real life most people have friends. In novels, even minor characters have to earn their keep by contributing to plot/character development etc.
If the plot is based on something that happens when ten old school friends meet for a reunion on an isolated island, then the friends are baked into the plot, but you won’t then have those ten characters phoning their other friends from different parts of their lives to tell them what’s going on, because it dilutes the plot and introduces too many random new names. It’s why PD James characters are almost always very isolated people, apart from the context in which we meet them around a murder (a private clinic, a religious community, an island where important public figures holiday) — she only wants to show them in a single context.
The novel I’m writing at the moment has a main character with no one she’s close to other than her sister, her DH and her married affair partner, who all need to be there for reasons of plot. When I need her to be out in a social context for plot reasons, I send her to a birthday party or an evening work event. Her character wouldn’t work or do the things she does if she had a gang of close friends she was frank with. She does talk to a graffiti tagger she meets out on early runs, but again, there’s a reason for that.
Even classic novels do it. Jane Eyre, despite loving and respecting her former teacher, later colleague and friend, Miss Temple, never hears from her again by so much as a letter after Miss Temple marries. It’s quite weird, but the plot needs JE to be alone and friendless, without sensible advice.
TL;dr. Friends in a novel are needed to play a role for plot/character purposes, otherwise they’re not there. Or genre purposes. Chick lit will generally have a stronger focus on friendships than, say, crime.