Well, there's a couple notes there:
1.) Graphics engines are actually a very small part of what an engine is. You need networking, AI, lighting, audio, streaming, level building tools, AI planning tools, asset management tools, physics, debugging tools on a per feature basis, scripting tools (preferably visual), concurrent editing tools, performance profiling tools, I/O tools, and more.
2.) There is an argument to mostly unified tools across a product portfolio. However, usually you're trading off some visual quality/performance in favor of more functionality/development efficiency. Like for EA it's far more important that Battlefield and Dragon Age can have 5000 features and seas of content than it is for them to look better than The Order 1886. For Sony, since their games tend to be notably smaller in scope, they tend to focus on things like best in class visuals to show off the horsepower of their system. This is why similarly you see all of Rockstar's games built on RAGE (so say, Red Dead 2 can use all the advances from GTA 4 and GTA 5 can use all the advances from Red Dead 2 and Max Payne 3) and almost all of 2K's games built on Unreal Engine. It also makes transferring staff between projects much easier. Like 2K Marin and 2K Australia could be pulled in by Irrational to finish off BioShock Infinite.
