You figured it all out at the end of your post. A lot of these developers are scum with no intention of building anything. You buy a plot of land for a million dollars in downtown, propose a 50 story Skyscraper the city can only dream of. The city enthusiastically approves the project. Now the land explodes in value at the very least because of the potential. Now they sell the land for many millions more based on the new speculative value of what has been approved.