It's just easier to tweak the game for console, because their is only one possibly SKU to deal with where you would have malfunctions (per platform). If I tweak something to improve performance on an Nvidia card, what will that do to those on AMD? If I load my game full of high resolution textures to make it look better, what happens to all the people on a GTX 770 with 2GB of RAM?
It's not so much that we tweak stuff to make it better on Xbox or PS4, it's more that we can tweak it because we don't have to worry about ruining the game for everyone else. We have a clear understanding of if the tweak improves performance for everyone, or it doesn't.
I'm sure this is the wrong place to say it, but I really feel the PC market could use some consolidation. For my last project we were fixing bugs that caused the game to crash immediately - only on AMD Phenom or older CPU's when paired with an HD5000 or older graphics card. When you are dealing with those kind of issues and ensuring your game actually runs on it, how can I optimize my game for a 940MX while running an i7 mobile CPU? Or maybe I should optimize on a GTX 580 with AMD FX8300? Or how about an i5 6600k and R9 290X?