I've seen a lot of folks talk about Sony's first party offerings and it leads me to believe that the launch lineup may be ridiculously strong.
It's wiser to wait as they have been and say "launch window" or nothing, so as to avoid any unfortunate scenarios if they are planned to be launch day titles.
As we get closer, maybe one reason is they are waiting on those guarantees from the developers. "Can you make launch day? Are you certain?"
Once they commit, slap the real date on it. It is wildly improbable, but think about the potential scenario here--if Ratchet, Spidey, Demon's Souls, and GT7 make launch, there's very little Microsoft could do to counter that outside of price, maybe. This doesn't include any surprises they may have kept secret til now.
Before you say that's impossible, *look* at how tight the info on the system has been until THEY spoke, predominantly. To say it couldn't be the same for software is also true.
I dont believe it'll happen, but if it does, whew.