It’s because not everybody knows how everything runs the UE4 (e.g I dont know how good the radeon HD 4870 is, but I know what type of Nivida card you will need to run the UE4 with a good fps rate) -> it’s important how everything fits together. But as already mentioned, when you can run those games, you shoud also be abel to develop with the UE4.
Otherwise go through this list and search for a similar setup : https://forums.unrealengine/showthread.php?20643-Official-Hardware-Performance-Survey 