Yes, but the SS is very low-stakes, no-danger, secret society, kids snooping on little local mysteries, aimed at younger readers. The worst that happens is the naughty sister Suzy getting hold of the password.
When the stakes are higher, and child characters need to be in actual danger, ‘good’ parents/ teachers/authority figures need to be dead, absent or rendered ineffectual. So the Famous Five are always off on holidays somewhere remote alone, or Jack, Dinah, Lucy-Ann and Philip get separated from an ineffectual guide, or Kevin McAllister’s family go on holiday without him, all neighbours are away bar the scary old man, and he’s afraid to go to the police because he accidentally shoplifted a toothbrush, or Dumbledore has to be killed off, otherwise there’s an actual adult (kind of) standing between him and danger.
In adult fiction, some genres allow for more concentration on friendships than others. A lot of women’s commercial fiction focuses on relationships between characters, so friends will feature heavily. YA often has a big focus on friendship. Coming of age novels often involve the protagonist finding friends/their place in the world.
Sally Romney’s three novels are all about friendships and how they interact with romantic relationships.
Becky Chambers’ ‘cosy’ sci-fi is all about friendships — pretty much all of her novels are about interspecies friendships on spaceships or planets. The Long Way to a Small Angry Planet is less interested in whether the Wayfarer successfully completes its mission than how a bunch of different species interrelate on a year-long voyage. The Monk and Robot books are about a friendship between a human and a robot set in a Workd where robots developed consciousness and left the factories to live in the wilderness. A Closed and Common Orbit is about the friendships that emerge when five individuals from different species are stranded for a few days at a refuelling stop.
What I’m saying is that if you want novels that focus on friendships, they exist, but some novels will have a reason for their absence.