PS3 sales:80 million units plus.
PS4 sales: MAYBE 3 million units.
if you dont understand math, the PS3 sold more units than the PS4. Which means more people will buy a game for that system this year. Why on earth would you ignore 80 million people just to take care of a few? (in comparison)
Everything you need to know about the PS4
It takes time for the transition to the next generation to happen. Sony didn't abandon the PS1 when the PS2 came out, they didn't kick the PS2 to the curve when they launched the PS3, and they aren't going to ignore the PS3 now that the PS4 is out. It's a slow transition that occurs over a good 2 years. MS gave up on the Xbox once they announced the Xbox 360, Nintendo kinda did the same thing with the GameCube when transitioning to the Wii (same with the Wii to the Wii U) but Sony has never just dropped support of one console outright. Even with the PSP, there was a transition period over to the Vita.