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.
Money you got from your retirement in pension is not closely to your full salary when you still worked in office. If you had very good job before retirement, received very high salary, you will get good pension after your retirement.
If people had bad jobs, low salary, they will receive low pension after retirement. It's opposite pension rate and can make life of those elderly a little bit harder.
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.
Some elderly get back to work because they love working and feel boring when they stay at home too long but not all people get back to work are like that. Many people have to do this because of money, they need money.