It's because there's a difference between taking an engine, making a level (or world in this case) with already created units and creating your own engine (or heavily modifying one) to make a unique experience. Ever wonder why so many Korean MMOs look the same? A lot of it is because they probably only make minimum modifications to their engines. Western MMORPG developers heavily modify their engines to make a unique experience for their games because their programmers are capable of such a thing. But I mean, even I could go download an engine and make a game world that looks beautiful by myself.
I don't think everyone posting in this thread fully understands what an engine actually is. It's not hard to make something pretty. It is hard; however, to make something unique, pretty, and well programmed.
e: Anyone interested, here's UDK: Unreal Development Kit (Unreal Engine 3)
http://www.udk.com/ You can try it for free.