Some may be working because they don’t like sitting around after retirement. My FIL worked at a hospital as an orderly after he retired from chemical engineering. Was still doing that in his 80s when he died. He liked being around people and helping others and staying busy. His hospital job gave him the chance to do that. Plus, he drove my MIL nuts when he was just hanging around the house with nothing to do. He started critiquing her housecleaning.
For the life of me I can't understand this. People working literally till the day they die? Sheesh, get a hobby!