Why does each radiator have an independent riser?

Hi. Our house has two-pipe steam. Love it. One thing I've noticed, though, is that on the second and third (finished attic) floors, each radiator has its own dedicated riser from the main in the basement. Some of these radiators are close in proximity (two radiators on opposite sides of a wall, but only a foot apart). Wouldn't it be more energy efficient (and easier and cheaper to install) for radiators to share risers? There must be a good reason why, or the original Dead Men wouldn't have gone through the trouble.

Second quandary: In the 1920s, the family added a new bathroom on the second floor and installed a new radiator in it. Despite there being a capped stem off the main close to the chase for the new second-floor bathroom, the Dead Man ran the riser back to the boiler and tee'd off the main where it left the header. (I'm thinking, 'Geez, the boiler has to prime that whole length of riser separately from the main... why not just share the main?'). That new radiator has never worked. The room is above a porch, so it's colder than the rest of the house (despite its insulation), and I imagine the pitch has sloped over the years. Condensate builds up in the supply line and the steam cannot get through. Maybe the radiator started out on that stem I mentioned earlier, and when the homeowner complained about the radiator not working, the plumber re-routed the riser back nearer the header to maximize the steam supply? I dunno. It's a mystery.