Because if there's one thing the internet needs is another music service. CNBC was the first to run with this and other outlets are now confirming as well that Facebook will debut its service during the f8 conference that starts on 9/22.
CNBC speculates that Facebook could function more as a portal than a massive streaming service or locker service:
Checks within the music industry suggest that Facebook has not been negotiating to actually become a music retailer itself; instead, it may be creating an environment where third-party music services such as Pandora, Spotify, MOGand others can offer their wares. If that's the case, rather than threaten existing music services, a Facebook music platform could actually provide a boost in traffic to those that offer free, ad-supported services.