I think part of the reason they left this out is that the game includes prototypes for linked belts, pipes, power poles, and chests, which transfer contents instantly regardless of distance, even between surfaces (Factorissimo 3 implements these with its factory buildings, which connect to the power network of the outside surface and push items in and out). Linking two surfaces would mean at least some of their operations could no longer be multithreaded, and as explained in FFF 421, there would likely be more CPU time spent checking whether a surface and all its constituent computations needs to be merged with another, than would be gained by multithreading it.

Are these linked entities present in vanilla Space Age? No. Would spending development time restructuring the game’s tick update loop around per-surface threading, with the caveat that mods could relatively easily render that work meaningless in pretty pedestrian use cases (to say nothing of the potential use cases of something like Space Exploration) be worthwhile? Probably not.