Hard surface amateur here. I notice guns (and vehicles to the same extent) are popular to model because they're fairly complex objects that offer good challenges (mesh-wise) while having a lot of references available (pick any gun, car, or military vehicle, and you can find tons of photo references detailing them under every angle). On top of that, they're common videogame subjects so it certain attracts people to modeling them.
For rooms, I'd guess the big appeal is from archvis. And most of the photo-realistic "looks like a picture" renders people see are of rooms like these, so it does create an impact of "I wanna make that cool scene too". And if you're gonna model something photo-real, you've got either interiors, exteriors, or props, and exteriors are way harder because of the organic elements in them. What else would you want people to model?
tl;dr there's nothing wrong with these subjects, if anything they're the things you should expect from people learning 3D. "Original" creations require a ton more expertise, and are harder to find good techniques/documentation for.