Because the card that came with it said it required validation by presenting a DL. It refers to finding a 24/7 location to do this, but many of the urban ones are unmanned and the website is not particularly helpful in indicating which locations are manned and which ones are not.
In the UK, you can ring up the Hertz 24/7 customer services and they'll do a conference call with the DVLA (Driver and Vehicle Licensing Agency) to confirm your license details, then they activate the fob for you. Is there not something similar for the US?
Another option might be to go to a manned Hertz office at a quiet time. If they can access the Hertz 24/7 system, job done! If not, you might be able to ring up customer services, then have them speak to a member of Hertz staff at the location and have them confirm that they have seen your license. Don't know if that would work, but if you're passing one it might be worth a shot!