The problem is that they are sitting in the middle of a wildlife refuge, and doing any demolition would actually cause far more damage to the local environment than simply leaving them in place. This is both in terms of simply hauling the demolition equipment in and trying to "rehabilitate" the land in some fashion after you have cleared away the mess.
Besides, there is always the possibility that some of those sites could still be reused, and concrete poured in the past for a launch pad is often very useful for subsequent launch site. For instance, the landing pad site at KSC that SpaceX is using to recover the Falcon 9 1st stage components is a former launch site that SpaceX got permission from both NASA and the USAF to clear away the metal on the site and set up the other things (like a radio beacon for the core to find) that needed to be put into the site as well.
Otherwise, the land is not really all that useful and can't be used for anything other than a place to study wildlife or launch rockets. Certainly no commercial businesses or homes can be built in the area unless it is directly in support of launch vehicles themselves. There is nothing else for it to do other than rot away, which has other very useful value in terms of trying to see what actually stands up to the environment of Florida over time and what doesn't.