Because Microsoft says they are? PS2 + ps3 combined still outsells the 360, month over month in the US. The Ps3 alone doesn't, but they still sell the Ps2 and there are still games coming out for the Ps2.
Across the globe, Sony isn't doing bad at all. Sure, I think they really would have liked to be outselling the 360 with the PS3 alone at this moment, but alas, it didn't happen. Big deal, they keep on selling the ps2 and look, it still works. Not everyone looks at the US as the center of the world: the rest of the world also counts, hell, in south america for example, all next-gen consoles are simply too expensive for the majority of the people.
Sony makes money for activision, so why should activision drop sony? It makes them money. So dropping the platform would LOSE them money, so they won't do that. Sony also knows that and I'm pretty sure they don't even pay attention to this nonsense.
The problem is more, like your comment, that a certain group of people think sony is in a tough spot and really on the brim to keel over and keep on repeating that whenever they can. That's the real problem for sony at the moment. Not activision's CEO whining about some licenses he has to pay for (which prices he knew up front).
Though Sony has the solution for this in-house: their own large group of 1st and 2nd party studios. The games they're creating and will create in the coming 2 years are going to make Sony enough money and will sell enough consoles that people who keep on repeating that they're in a tough spot and ready to keel over are not believed anymore.
In that light, MS has bigger problems. But that's an umpopular statement in gaming world with the large group of very vocal 360 followers online. ;)