me_irl

they might be doing it because they are bored

Both/And. Retirement is boring. Retirement when you're broke is absolutely enervating. Menial employment kinda-sorta solves both problems. Although, its as much a pox on the employer as the employee. Elderly workers don't tend to be the most motivated or the most energetic. And when you're paying a pittance, they don't want to bend over backwards for you, either.

my grandparents both volunteered and did odd jobs to keep active in retirement and lived until their early 80s thanks to that, they had plenty of money.

I mean, "plenty of money" is sort of a YMMV situation. I've got two in-laws both with less than a million in savings. One lives frugally to the point of a asceticism while the other just seems content to YOLO until he's down to whatever SS has to offer. Idk how much money they're going to need in another ten years. Nevermind another twenty. But they're both in an inflationary vice that keeps squeezing tighter with every year.

My own surviving parent spends her more generous retirement savings endlessly fixing up the four bedroom house she refuses to sell, in between routine visits to the doctor to find a cure for being old. She might actually benefit from getting out of the house to do some bullshit retail work, except she's more of a management-type personality than a worker bee. I'm not worried about her finances nearly as much her mental health. But I could see a future in which she's suckered out of a big part of her fortune, simply because someone on the TV sold her on it in a moment of weakness.