In my country, people who are of a suitable age for retirement, being already retired, prefer to stay in the ranks and continue to work, but only for half their previous salary. For many, this is a necessary measure since there is not enough income, and for people who are accustomed to constantly working, that is, producing something, it is difficult to remain idle. But I agree with many who argue that retirement with a healthy financial cushion would be preferable to a constant workweek schedule.
If you are retired you will be receiving a pension which is close to your salary or even more that is why are others really wanting to retire. I'm not sure where you live but mostly I've noticed that employees want to have an early retirement as their pension is close to their salary and they are not doing anything.
Most of the reason why people are back at work is that they are just bored and they are used to doing work this is like your grandmother that they can't just sit and watch TV all day as they want to clean the house and do other stuff, so that is also the feeling of retirees.