Even if you provide optimal documentation, it's got to take a real person a very long time to go through the receipts. And I'm sure not everyone has optimal documentation (everything is legible, sorted in order with a list, etc). I pulled a few dollars in the first year (2014), just so I had an example of the form, but pulled none since. The juice is the time between depositing and withdrawal for medical expenses. This year I'm pulling the whole of 2014. I was good at record keeping in those early days. As the years went by, I just have a receipt file that's not bound and not reconciled to a list; just reports from my accounting system and a pile of receipts. A PITA if I'm required to produce documentation, but I could do it.
But got off on a tangent. I meant to address the complexity the IRS faces. Say you submit stuff over several years, no CP letter. Then you get a letter and provide receipts. Then, in a later year, another CP letter, you send in more receipts. To be thorough, the IRS would have to pull the prior list, and make sure you didn't use any of the same receipts. What a nightmare for the IRS.