Mostly to do with floating point precision, the devs of Star Citizen ran into the same issue with Cry Engine when they tried to make things space-scale, and ended up initially stuck with small battle-arena maps. UE4 can re-origin things for a single player moving through a large world to maintain physics precision, etc. but it doesn't currently work with multiple people and their server model.

So, basically what Kerbal Space Program does. Which also doesn't work well with multiple people.

That said, keep in mind that you can get roughly a maximum of 20 sq km with the default landscape tool (not sure about performance at that scale).

I believe there's also some hard coded limits around the number of simultaneous players (16?), though it seems fairly easy to work around. Again, not sure about performance.

Nothings really hardcoded when you have the source at hand.

True. Also, there are rumors that epic is working on a multiplayer implementation of the player centered coordinate system.