I think it's the sheer forbidden nature of it that draws people in. Like, here's Homelander, this terrifyingly powerful god-complex-in-a-cape, and Hughie, arguably the most 'normal' guy in the whole show, just trying to survive. The power imbalance is astronomical, which in fic terms becomes this playground for exploring vulnerability and control from both sides. Writers get to dissect Homelander's loneliness and Hughie's quiet resilience in ways the show only hints at.
It's not about romance in a conventional sense for most of these stories. It's about corruption, maybe. Or about Hughie's innate decency acting as this weird, improbable anchor for Homelander's chaos. The appeal lies in the 'what if' - what if something in Hughie's genuine, scared-human reaction actually registered with Homelander? That tiny possibility, against all logic, fuels a thousand AUs.
Plus, let's be real, the actors have chemistry that crackles even when Homelander's being a psycho. That tension translates.