Which city is that? Most cities are unwilling to sell off public parks to private developers. Do you have specific examples of parks that were built over and not replaced with something accessible to the public?
In South Australia there was recently protests to stop a parklands getting turned in to a supermarket. I never heard what ended up happening there but the council was pushing hard to get it converted because "Shoppers are demanding more choice"
I'd rather not be too specific, but in the lower mainland in BC. Yeah, the one that used to be just down the road from me that's about to be turned, first into a giant pit, then into a large complex of highrises.