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.

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.