What makes mmos hard

Most mmos are rpgs. And while rpgs have a lot of stats and items a lot of code can be reused and art doesn't have to be crazy. So that doesn't seem to much harder than anything else .

Ok so then there's the online aspect. Obviously this is a can of worms. Is that what everyone means? Or is it like regulating cheaters and hackers.

I don't see why mmorpgs are like the single thing you shouldn't try to make.