Well @title, I have an answer! Games are so modern that they push the extent of the system to its limits, and they often have to find ways for it to run singleplayer to begin with without dying. When adding a second player, you essentially almost double the load, double the rendering, double the calculations, and double the pain on your console. It'd never run, never, it'd just cry and plead you to kill it and its 2fps.
It's really not that hard to reduce load.
Turn down texture quality.
Reduce scaling.
Reduce anti aliasing.
Uncharted 3 did it perfectly fine, and that was a very stunning game for its time.
Even now, star wars battlefront (the first one) is split screen, and still looks good.
It's not that difficult to do for developers
For the record, COD: Black Ops 3 has split screen support even on PC. You just need a controller (PS or XBox) that you can plug into the pc along with the controller. You can also play with 2 controllers.
There are some other games, such as Cuphead, which have local co-op support, once again with keyboard & controller or 2 controllers.
It's also directed by Tommy Wiseau ... The Room guy
Turn down texture quality.
Reduce scaling.
Reduce anti aliasing.
Uncharted 3 did it perfectly fine, and that was a very stunning game for its time.
Even now, star wars battlefront (the first one) is split screen, and still looks good.
It's not that difficult to do for developers
There are some other games, such as Cuphead, which have local co-op support, once again with keyboard & controller or 2 controllers.