Short answer: the two platforms are still worlds apart and porting/mutliplatform development is still a ton of work.
Longer answer: the platform use different rendering APIs (basically the whole low-level rendering core might need to be rewritten), different network and OS APIs, have their own TRC (technical requirement checks, which are hundreds of pages of per platform things that need to be done), you have to match the QA, you still have very different hardware configuration (especially when it comes, you have to create platform specifics shaders (and compilers). Basically, it's still thousands upon thousands of hours and depending on what the studio(s) are doing and what their expertise are, outsourcing the technical work can make a ton of sense for all involved.
EDIT: Look at like this: technically the generation has been a very tough for developers (for a multitude of reasons, which I won't go through here nor can I disclose fully the details). CD has not shipped a single title using the PS4 development environment and especially the GNM/GNMX rendering API. They have not gone through the usual cycle of failure and iteration Avalanche has whilst shipping two titles between its studios. That means Avalanche are actually much more likely to ship faster and better given the same resources. Of course, S-E would likely help with sharing their learnings, but I highly doubt they are nearly as good in knowledge sharing between studios as Ubisoft for example (mainly because they don't own all the studios).