It's not like they're prairies that you can file a homestead claim on. They all are owned by developers, some of whom paid too much for them. That, and other factors, complicates their development.
The one nearest Willis Tower, at Harrison & Wells, is the former site of Grand Central Station, and is sometimes called the "Franklin Point" property. For decades, the railroad's real estate subsidiary held on to it, because their carrying costs are low and they thought they'd eventually make a killing. Finally, in the last boom, the property was sold at high prices to a couple of small-time developers with outsized dreams. But the crash got here before the bulldozers did. One of the purchasers just tried to get the sale rescinded because the property's value has dropped by a third since he closed; the court just laughed at him. It's too far from the West Loop train stations to be considered a prime office location, and too far west of Grant Park to be considered a prime residential location.
The one south of River City is owned by Cacciatore & Co. Real Estate. They invest for the long term, and during the next boom or the one after will try to make more money per square foot than anyone around them.
The big one south of Roosevelt Road, sometimes called the "Riverside Park" parcel, is problematic for development because it has no access or infrastructure. So you have to build out a big portion of it all at once—a huge chunk of money. A plan was approved during the last boom, but then the main developer got enmeshed in a local political scandal and sent to jail, and as the boom ended he flipped the property to others who don't really have any development experience.