Why does certificates have an expiry date?

In order for pilots and cabin crew to maintain knowledge and be eligible to fly, there are certain requirements that have to be met.

The certificates that are generated are mainly to confirm that training has been completed. Some of the training is one-time and therefore that training does not require an expiry date.  

We know that recurrent training improves pilots' proficiency and increases situational awareness so when an expiry date is added to a certificate the purpose is to increase the safety of passengers and crew members.

The expiry date on the certificate of training does not affect the Pilot certificate per se, but it is a way to make sure that the Pilots who fly have updated knowledge to be well prepared for any situation.