Why do we demolish buildings instead of just deconstructing them?

In addition to the costs mentioned in other responses, there is the issue of the materials that make up the building. These materials have been altered and possibly ‘stored’ in conditions which may alter their attributes. In order to reuse these materials they need to be reassessed and possibly tested. When compared to the ease and cost of new reliable materials it commonly doesn’t make sense to reuse much of the existing materials in any other capacity.