I still don't quite get why cities should be any kind of enclosures. The world as we know it has these huge air filtration and CO₂-to-O₂ conversion systems call

I still don't quite get why cities should be any kind of enclosures. The world as we know it has these huge air filtration and CO₂-to-O₂ conversion systems called "forests", and I do not understand why cities should be disconnected from them (apart from perhaps protecting the forests).