DirectX and XNA will never turn theirselfs into engines, because they're just library's. The game engines are based on DirectX and XNA, and why do you think XNA is low level, many games on the xbox are written in XNA. Game engine's are just there to ease the programming, but they are still based on library's like XNA or directX, and if you're looking for flexibility it's better to write your own game engine, cause with other engine's you will encounter stuff that you would like to see differently or not at all