City planners would if they could, but they're not autocrats. They know that green space (and mixed zoning and high density and...) is a good thing, but they are captive to the whims of politicians and poorly-informed citizens. And property developers have more than enough money to buy policy and sway public opinion.

A close relative is a city planner, a job he went into as an idealist. He saw the economic, social, and ecological suffering created by terrible urban design, and he wanted to make a better world. After 30+ years, though, he's jaded and cynical.

He's seen historically active flood and landslide and earthquake areas quietly and repeatedly re-zoned for residential development. He's seethed at 40 years of opposition to even modest expansions of mass transit. Suburban sprawl, ghettoization of the poor, retail-only developments... At this point, although he still does his best to advance healthy policy, he's resigned to the fact that the developers always win in the end.

So please don't direct your ire at city planners. In many places, they're the only resistance to the boundless greed of property developers. As in all things, it's important to follow the money. The only people getting rich off the status quo are banks and developers.