Uhm... Why do you ship Stancest they're literal brothers!
Hi, anon! Thanks for asking this! While I don't owe anyone an explanation for the things I choose to like or enjoy, I'm going to give you my genuine answer. If I receive further asks with negative tones or connotations on the matter, don't expect a response.
Back when the show was coming out/finishing up in 2015ish, the common main ship for the fandom was Billdip (as reflected in the AO3 most-populated relationship tags), which left the Stans without anyone to be shipped with. I'm a multishipper, so I played around with fiddlestan/fiddauthor, but since learning the Mystery Trio wasn't canon, it had less appeal. The Stans already had a complex relationship, so expanding on that wasn't too uncommon. The ship wasn't exactly popular, still, but it was also quietly recognized by some fantastic fanfic authors for its potential, too.
Even so, your question is something I asked myself quite often, too. I wasn't really certain why the idea of Stancest appealed to me. I'm certainly not interested in Mabel/Dipper stuff at all, but for some reason Stancest felt more interesting. I think @huginsmemory explained it best.
"As many well spoken posts on Tumblr (such as these) have talked about, most… incest fantasies aren't really about that; instead it's partially about someone loving you so desperately or finding you so sexually attractive that they will transcend typical boundaries and expectations set within society to be with you no matter what (in the safe environment of a fantasy). It's similar when you talk about something that is positioned to be inherently dangerous and uncaring; monsters are not supposed to care about you, nor are supposed to be gentle. And yet all monsterfucking stories are of the monster in some fashion caring about the protagonist, whether just being sexually attracted to them or full on being in a caring relationship. It's about how the monster can't help but care about the protagonist so much that it changes their (perceived) nature, of the monstrous becoming somewhat humanized; even if only around their love interest." ("Stanford Pines; a Case Study on the Aro-Ace Spectrum, Monsterfucking, and Their Intersection")
This post explores the intricacies of asexuality and monster-fucking in relation to Stanford Pines, but helped me understand some strange things about myself, as someone who struggles with their asexuality and the complexities involving it. For me, it really hit the nail on the head. Fiction is a safe environment to explore fantasies, especially fantasies of things I would never want in real life. (I would never condone irl incest, and I would certainly not be comfortable with a monster trying to fuck me irl, either.) As someone who feels broken and out of touch with a society that values sex and sexual relations, the idea of someone wanting you so, so deeply, no matter the obstacles or societal boundaries, is very moving to me. And because it is so moving, there's an added layer of eroticism and intimacy that becomes accessible to me.
(Note: I'm not saying asexuality is a broken sexuality, or that all asexuals feel that way, or even agree with the things I'm saying. This is just how I feel, and my thoughts.)
All of this being said, you're pointing out something I used to enjoy from over a decade ago now. My likes have changed. I still appreciate the ship and the excellent content I found with it back in the day, but it's not something I actively find myself engaging with anymore. In addition, everyone has a right to enjoy and explore different things in the realm of fiction and fantasy. You don't have to like or agree with the things others do, and it is okay to ask if you can't fathom why someone like something, but no one has an obligation to explain themselves. I'm choosing to, however, because this broached on something I've been thinking about a lot lately (Why did I like Stancest? Is that something I would engage with again? What does this say about me? How have my preferences have changed over the years?), and because if this answer and this post can help you or anyone else feel better about strange things they enjoy, then good!
Thank you for asking, I hope you were receptive to my response and that it helps you in some way!