Yeah this is something I've noticed too and never really seen it talked about properly. Every credible account I've read, the figure is either in a doorway or at the end of a corridor leading to one. Rarely in the middle of a room.
My theory is it's something to do with thresholds - like doorways represent a boundary between spaces and whatever these things are, they're drawn to that liminal point. Some older folklore backs this up actually, the whole idea of spirits being trapped between places rather than in them.
Spent a night at a reportedly haunted farmhouse outside Dundee a couple years back and the one thing we caught on camera was a dark mass that appeared right in the frame of the kitchen doorway for about 3 seconds then gone. Nothing in the middle of the room, nothing in the corners. Just the doorway.
Anyone else got examples where the location was specifically a threshold? Would be good to map out whether theres a pattern here across different types of locations.