@sidekickin
The reason you need to account for depreciation in this way is because the tax code requires it.
as to why the tax code requires it, section 1016 was rewritten along with the rest of the tax code in a major rewrite in 1986, but parts of it were in existence as early as 1913. You would probably have to find an expert in accounting theory and history to explain why Congress decided to treat depreciation in this way.
The bottom line is that when you sell property, you must take into account all depreciation that was allowable over time. There is no exception if you chose not to claim depreciation that would have been allowable when you filed those prior tax returns.
