Many industry-standard courses require certification to expire. Mainly, expirations exist so that learners refresh their knowledge every few years, which is important to do so in order to be safe on the job site. Furthermore, rules and regulations may change and legislation may be passed, so a course from 3 years ago, etc. will likely be different today.
Please note that many different courses have different expiry dates. If you're wondering how long a specific certificate is valid, you can find this in the details on the course page.