Because they're open-sourcing an existing library?

To answer the larger question of "why create a new library instead of contributing to open-source project X instead", the answer is that working on other peoples' open-source projects (if you're actually going to improve them rather than just throw more me-too mud at the wall) usually consists of cleaning up other peoples' messes. You're at the mercy of a third party maintainer, you're stuck with someone else's (probably terrible) design decisions, and you're dealing with a list of existing bugs and defects.

I can't speak for everyone but just personally, if I'm doing a job like that, it's because I'm being paid for it.

They do: http://developer.amd.com/community/blog/2015/07/10/collabora...

Because Cycles is a rendering engine, while FireRays is a ray intersection acceleration library. Cycles could opt to use FireRays for ray intersection if they wanted to, but most uses of FireRays probably could never use Cycles since need a rendering engine with entirely different capabilities (real-time &c.)

I mostly work on closed-source software. If they contributed to Blender, I would never be able to use that FireRays because GPL.

Instead, AMD released FireRays under permissive license, so I can pick the technology for a next project if I see fit. BTW I have good experience using libraries by AMD, such as SSEPlus (Apache license).

Depending on what you want to render, and what kind of look you are after. Blender cylces can be the worst of raytracers. I'd be more interested in a general ray-hitting lib, less then a raytracer.