For the same reason it's the public responsibility to provide infrastructure to get private property from A to B (roads), protect people's private property (police, military, firefighters), etc. It's just infrastructure.
There's a common benefit to having spots available for businesses and other uses. Its sort of why we have parking meters in the first place, so people don't camp spots all day long.
I get this. But this isn't a community problem. It's a community choice.