It's not like they haven't thought of it, or even spent money on it. From the Bureau of Engraving and Printing:
"Also, a machine-readable feature has been incorporated for the blind. It will facilitate development of convenient scanning devices that could identify the denomination of the note."So the blind have to use an expensive electronic device to scan the bills, which they FIRST HAVE TO INVENT, instead of just reading a few bumps that could be impressed on the bill by running the proofs through an embossing roller.