Middle-earth: Shadow of Mordor's Nemesis System was patented by Warner which is why it hasn't appeared in more games
The reason you haven't experienced the Nemesis System from Middle-earth: Shadow of Mordor and Shadow of War in other games is because Warner Bros. patented it.
The patent details the Nemesis System built for both games and was found by Game Maker’s Toolkit and discussed in the latest video which you can watch below (thanks, PSU).
If you are unfamiliar with the Nemesis System, it basically uses memory techniques, which means the game can remember actions performed that become part of the narrative. In turn, enemies in the game will remember what you have done the more you come into contact with them.
In the Middle-earth games, named orc bosses that you kill will have their abilities passed down to another orc. But if you die at the hands of your enemy instead, said orc may rise up in the ranks with new abilities and will come after you again. Orcs will also fight each other to rise up in the ranks.
And because it has been patented, we will probably never see something exactly like it in other games. Maybe something similar, but not the exact system. For now, the closest thing we have to it is Assassin’s Creed Odyssey's Mercenaries system.
As RPS points out, this isn't the only system to be patented by a game maker. Bioware patented its dialogue wheel, and Take-Two filed a patent related to AI navigation developed by Rockstar.