Well, why dont we "create" a masterblock.
A masterblock is created by the miners every 1,000 blocks. It contains all known bitcoin adresse and their volume at the moment when the block is created. The whole network checks the masterblock and give its confirmation if its legit and right calculated.
Someone comes up with this idea, or something like it, about every six months or so. The answer to "why not" is that we don't need to. There are other solutions to the size of the blockchain, most notablely the one that was proposed by Satoshi himself in his whitepaper. The blocks were designed to be "pruned" of long spent transactions, if the user desired that, but never to require such an action. That's why the internal block structure uses a Merkel Tree instead of a flat file format. Including special blocks to update a running list of non-zero balance addresses would not only require breaking the current system, it would also eventually be blocks of such massive size as to be counter-productive.
There is no full client that utilizes pruning that I know of, but there is nothing preventing you from coding that yourself if you have the skills and believe that pruning is a near term need.
Furthermore, anyone who desires to avoid the blockchain altogether can simply use a light client, such as BitcoinSpinner or Electrum.
Search the forum for "pruning" of blocks, light clients, and "Stratum" for more information.
