The problem with Massa is that while he is a good No.2 driver, when it comes to taking on the likes of Vettel, Alonso (when he was in a decent Renault or McLaren), Hamilton, etc, he falls short. This is why they brought in Kimi, who in fairness outperformed Massa to win the title in 2007. Ferrari know this. Ferrari also know that over the next few years Hamilton will probably dominate and to prevent this from happening, they need either Alonso (who they have succeeded in getting) or at some stage, if Alonso leaves, they will need to get Vettel or Hamilton. Massa simply doesnt have the weaponry to handle Hamilton, in a similar specced car.
If push came to shove, Ferrari could get rid of Massa and hire another decent No.2 driver (Webber comes to mind). Furthermore, Massa will also know that he has it very nice at Ferrari and leaving Ferrari really would have to be a last resort.
The point I'm making is that while there are plenty of decent No.2 drivers, as of right now, there are probably only 2-3 decent No.1 drivers, meaning that a team has to give preferential treatment to their lead driver, or face losing him to a rival team.
If push came to shove, Ferrari could get rid of Massa and hire another decent No.2 driver (Webber comes to mind). Furthermore, Massa will also know that he has it very nice at Ferrari and leaving Ferrari really would have to be a last resort.
The point I'm making is that while there are plenty of decent No.2 drivers, as of right now, there are probably only 2-3 decent No.1 drivers, meaning that a team has to give preferential treatment to their lead driver, or face losing him to a rival team.