It's not the planners. It's the money. People want to live on the first floor and BBQ on a lawn and they will work like dogs all their lives to have it. The farmers can't make any money because the big stores buy from mass-production farms located around the world so the landowners sell their land for a one-time "cash-in". The government can't stop a private sale. They can create zoning laws but the politicians who agreed to zone land as agricultural can also vote to rezone it. That depends how much coin is offered by the developers. When they are offered enough, the bait and switch is played... we need the tax base because we need money and, if we don't allow developement, we will have to raise taxes.