Well it's obvious that they're using that extra money to invest into their new games.
/s
There really isn't a good reason, but since they are the market leader by far nothing will change until they feel some heat from rival stores. I expect console platforms to increase dev payouts sometime next gen. Perhaps the mobile devs will follow but they deal with much more different price points compared to console/pc so I can understand a delay until they figure it out.
Why, out of the goodness of their hearts? They of all people have no reason to change anything. They have publishers by the balls at least until the streaming future takes off, then consoles will look just like PCs do now but worse since it'll be streaming too. I don't look forward to that day but at least it will be nice to see most of this place change its views on this stuff.