Because historically that's how it worked, but officials just looked at the document and verified that it was the real thing. Then photocopiers came along and it became normalized to take copies of the documents. Then digital copies happened and that changed things completely when coupled with networking technology. What the officials in charge don't seem to understand is that by making digital copies in networked environments the IDs themselves lost their value completely, after all if the digital copy serves any purpose at all as a stand-in for the original then they have become that original.