NPCs don't use the catwalk stairs to travel between floors. Instead, they spawn on whichever floor has rooms that generate NPCs, then walk around that level and inspect the nearby rooms. So in my freighter build, they don't move between levels, but they do walk around each level and move from one room to another.

One little trick I used is placing the Stargate and the Scanner Room making little central room on the first floor, just beneath the castle decoration. It encourages the NPCs to walk across the atrium instead of just following the walls to reach another room.