Because they likely already have more than enough and have been blessed by society/civilization as a top earner who will enjoy a comfortable retirement without any social security, and they’ll be better off if other people that didn’t earn and save as much are able to retire without being destitute in old age.

That perspective could be someone who is willing to say “You know what, I already have enough, let’s make sure the floor is raised for everyone.” Someone who believes more in individualism would probably disagree with that perspective.