But why do they lower the graphics across all systems and not only for last-gen consoles ?
Because when changes like this are made, it's not as simple as lowerng a single value from 100 to 80 or something. It's usually a matter of completely rewriting the code for certain functions. Telling the engine how to write data to the RAM, and in what order to retrieve it in certain situations...that sort of thing. Once changes this deep are made, it's a terrible idea to take things in one direction on PS4, a totally separate direction on Xbox, and a totally separate direction on PC. If I do, every subsequent change will have to be handled differently on every different system. Before long, you'll basically need three completely separate studios to manage the three completely different ways the game is being designed.
Thus, patches will be made universally. In this case, changes to the rendering system will be a general attempt to more efficiently handle things, across the board. It's the same type of thing as, say, adding ray-tracing functionality to a game. I'm not going to write 3, totally different ray-tracing packages -- one for PS4, one for XBX, and one for PC. I'm going to write one, universal system that works on all platforms, then tweak them a little here and there as needed. If there's a major issue only on PS4, I'm not going to rebuild it from the ground up just for PS4, I'm going to edit the core functionality to try to get it working on PS4 without changing it too much for XBX and PC.