In development, you don't have the privilege of using optimised culling and LODs. For rapid development, very little code is compiled & there are extra IDE tools running.

When you consider that 16GB has become the minimum requirement for modern A+ titles (with the Windows OS & background tasks squating on 4-6GB). Creating such a title might be difficult on memory limited machine.

Unreal had made some improvements in this regard recently, with direct from storage assets loading & stuff.