Which ones? Most were developed into farms, cities and industrial zones. All directly responsible for pretty much every thing you're enjoying about your life.

Sure, it would be even better to have both all the forests and plains, and all the food variety, telecommunication, transportation, lifespan and individual freedoms. But it was never a possibility. There was no path from there to here for humanity that didn't mean developing much of the land, cutting down much of the forests and poisoning much of the world - it's because we were (and still are) figuring things out as we went along, acting in an uncoordinated, distributed fashion.

We could eventually restore much of what was; we can see the possibility now, but the path to it starts with developing Earth orbit. The way to get the forests and clear sky back without making life universally worse off leads through weathering the whining of astronomers as infrastructure, industry and habitation is being moved up the gravity well.

(It's either that or making some huge leaps in biotech, but that's pretty much post-singularity regime, in the sense it's too hard - or too scary - to predict how the world would look like after working with DNA becomes as easy as working with code. Expanding to space sounds like a much safer option to pursue first :).)