I understand the reasons behind wanting to shorten certificate validity periods, but CA or root certificates often have expiration periods far into the future. What’s the argument for this? Ease of use? Historical reasons?
Neither of those really apply to the root certificate store. Updating the root certificate (to add or remove CAs) requires an OS/browser update instead of relying on CRL or OCSP queries which are flakier--and browsers have a variety of mechanisms to punish misbehaving CAs that aren't "drop you from the root store effective immediately."
Meanwhile, the process for updating root stores is entirely different than regular certificates, and it takes months to years to add a new root certificate--and I believe you don't get a pass to merely update your new root certificate for an existing CA.
The other risks with overly long validity times are ameliorated in large degree by the fact that CAs are regularly audited and have much greater scrutiny placed on their issuance.
For this reason root certificates have a very long validity and very stringent requirements in their security parameters, e.g. large key sizes, and how they are used. Usually the certificate sits in a well secured off-network physical device and is only used a few times during its lifetime to sign sub-certificates.