Simple. Developers did not purchase the land but rather purchased the option to buy the land. Rather than pay 100 million to a land owner they might have paid 4 million for the right to buy at a specific price.
Rather than increase inventory in let’s say Del sur they have elected not to exercie their rights to buy that land. Instead of creating an additonal 1000 units of housing they elect postpone plans to build. their cost to walk away from the project is simply the cost of the option. They do not carry the same risk as had they bougth the land outright. Additionally they now have effectively reduced the amount of hiomes that will be built out in the short run.
