I would say it's a combination of all. The pension is good, albeit a little shaky the last few years. The main aspect is not financial; it is that pilots just love to fly. They tend to be a somewhat alpha group of individuals who worked long and hard to get where they got. They enjoy their work and would like to continue to fly.
In my own case, I would have preferred to fly a few more years with a retirement date of my choosing and then continue to fly my own aircraft, which I still do.
The legislation in human rights states that the choice should be up to the individual and should not be imposed by any other persons. This includes the tyranny of the majority. A large proportion of younger pilots would obviously prefer that the older ones retire immediately to increase their own advancement. But they may well feel different as they approach retirement age themselves.