The same reason everyone has a deep learning platform. It’s about developer mindshare and industry dominance rather than honestly thinking they’ll make a better framework starting from scratch rather than improving someone else’s.
Apple actually sort of does have a backend framework: WebObjects. They just seem to have deprecated it in favor of other solutions. Still, I've noticed at least iTunes Connect is still using it. And for what it's worth, Swift is going to have more server related stuff coming to the standard library (https://swift.org/server-apis/), and I wouldn't be surprised if Apple starts adopting this eventually as well as it continues to improve.
