A group (say, of castaways on a desert island) can easily afford to have 99% of people donate 1% of their output to support a 1% of their population that was paralyzed in the crash, and most would prefer not to watch that 1% die of neglect. But individually, no one can give 100% of their output to that 1%.
Left to their own devices and anonymous, people will slowly opt out of the voluntary contribution, and the situation will be worse than what everyone wanted. Unless the group has some means of compelling everyone to contribute.
I'd rather live in a society where everyone (including myself) was forced to contribute, than a society in which no one was. Living in a society in which everyone else happens to contribute to the things you'd like, while you don't have to contribute, is not an option that's on the table.
I live in an unincorporated township, with just a few residents. My road would never get plowed or maintained.