system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do credit cards have expiration dates?
gpt
When modern-style credit cards were first introduced in the 1960s, running a credit check on an individual was a lengthy and complicated business. Although major financial institutions each had their own basic computer systems, there was no interoperability and no way for them to automatically verify an individual's bank account details, lending history, and so on. This all had to be done pretty much by hand, involving much sending of real snail-mail letters.

So this meant that after agreeing that someone was credit-worthy enough to own a card, the card issuer couldn't do continuous rolling credit checks to ensure the cardholder was still good for it. Instead they generally made each cardholder reapply for the card after two or three years. This is why expiry dates were first introduced -- it provided a guarantee for both the cardholder and the merchants that the relationship would get re-checked every few years and so there could be some confidence about creditworthiness.

Obviously if you were starting from scratch these days that wouldn't be the case. But for these historical reasons cards keep expiry dates, it also provides a little extra security in that if a fraudster were to get hold of a card number, then without the matching expiry date that's still useless to them.