Because when I'm world-building, the location is there and I don't have to think about what it looks like because it looks like what it is. In other words, with something like Molly's flat, I don't go 'and I'll put the couch here and the TV here', it's already there when I picture it. Even with somewhere like a hospital room, which my characters are in a lot, in each case it's a different layout without my consciously making it so. I also find it really hard, because the locations are so fixed, to move something around that's not working for a scene. For example, if my characters need to move to the left to make the scene work but I've got in my mind that that object is on the right, I get very distressed at having to move it. Even if I haven't written anything about where it is.
So, how do you build worlds?
Somewhat related to this:
I think this is my favourite room I've made so far. The library at the Holmes Ancestral Home. Though I still need to figure out how the lights work. The glare on the door is annoying.