> Why not?

Because “ownership” is the right to exclusivity.

> Plenty of people own parks and gardens that are open to the public

You can choose not exercise the right to exclusivity (or to do so selectively), but if you don't have it, you don't own anything.

> Everyone getting to enjoy the thing while the creator still gets paid sounds like an awesome future to me.

Sure, patronage of public art can be great, but it's not ownership (except maybe “ownership” of an exclusive right to receive some defined form of credit for the patronage.)

Those things you have described are still exclusive in that the owner _could_ prevent public access if they chose to.

the control is exclusive. the owner can always put up a fence, or tell people to get off their lawn