It expires so the issuer can charge you more money for the next one, to add more security features, to update the photograph, to change the look of the card, to name a few reasons. For example, my Wisconsin's State ID used to cost me $4.00 and expired 4 years later on the date of my birthday. When I renewed last year, it cost $28.00 and is good for 8 years and expires on the date of my birthday. Doing the math, it cost me 7X as much as the last one and valid for 2X the period of time of the last one. When I asked one of the staff at the DMV why State IDs had gone up so much, he stated it was to comply with the Real ID legislation. I can't complain, driver's licenses are even more and if you had a DUI or accident, the cost is higher still.