Because you cannot rely on the rich to fund common goods through voluntary donations. It is not a good idea and never will be. Other than some medical research, charity has never solved a single social ill.
It is peculiar that the model of fundraising via newspaper ads and a couple of wealthy donors (including in some cases the national government, yes) doesn’t seem to have survived past the world wars, but to say that private funding never resulted in any social good is preposterous. Something has happened in the last century or so, but it’s trickier than that.
Calling a hoogheemraadschap a private enterprise is calling a government one. If it acts like a government it is one even if it is not benefitting everyone equally (still no difference though).
That you can not rely on the rich to put their money in good use with the general public in mind is right. Does it happen? Yes. Does it happen often? Definitely not.
(similar thing in education involved deciding "equality" was bad, so they invented "equity", which is the same thing except they like it.)
> Other than some medical research, charity has never solved a single social ill.
This is very broad and incorrect statement that I suggest you research more before stating as fact. Maybe start with libraries in the 19th and 20th century if you don’t know where to start.