To build a development you need planning permission. This will often include an agreement that public open space will be provided as an amenity for the community. Sometimes this will be as mitigation for loss of other space. Or just as a something the local authority wants. It is probably better than stricly gated communities that exclude the public community, which is what the councils want to avoid.

Also, the council may not have the funds to properly maintain the open space if it was handed to them. It makes commercial sense for operators to do landscape work themselves if that is something that matters to them. People want retail, housing developments or high end office blocks to be attractive and well maintained.