Because there's no way that they're going to take away a reason to own the PS4. They might give access to legacy titles on the PC through Gaikai, but that's about it. And they might even throw in a cutoff there (up to PS2 titles).
Hardware tends not to make Sony (SCE) a lot of money.
They want to to control and gate-keep your access to content, and owning the box you play through is one way to do that, but it's not the only way. You can be a platform holder via software only also.
The risk for Sony in a software-platform world is the ease with which consumers can jump between platforms vs a hardware-platform world, where your hw investment helps tie you to their content.
But...if more and more gaming and entertainment options that are hardware agnostic and far more convenient to access start coming on stream, Sony is going to have to compete with them anyway. But with higher barriers to entry for their platform, if locked behind a $300 box? Not a disadvantage they'd like to have, IMO.
It may be a case of cannibalise yourself or get cannibalised.
I could see Sony making PS4 a vehicle for software on 3 fronts. Digital download (to a PS4 box), retail discs (to a PS4 box) and digital streaming (to everything and anything). You'll still want a PS4 if you want the optimal AV experience. But I think Sony might explore compromises between quality of experience and convenience to broaden their audience, perhaps substantially.
IIRC Dave Perry said one of the reasons Sony acquired Gaikai is because their technology would work on standard web technology - and help get games into devices via browsers, side stepping app stores etc. I thought that was an interesting comment. If you look at their job posts they talk about bringing console quality games to everything from TVs to mobile phones. Sony may well bring 'something' to everything...and I think they will at the very least experiment with their new platform's content within its lifecycle, if not go the whole hog with the above 3-front strategy.