Why is a user's updated time being updated on a frequent basis, even though the user properties like name, custom fields, or identities have not been updated?
When any attribute is updated on a user, the Updated value will change. This includes updates coming from syncs with other systems (like CRMs, SSO integrations, and other user databases) as well as updates surrounding user logins.
Pertaining to user logins, every time the Last sign-in value updates, the Update value will also update like shown here:
The obvious case here being if you login using your user id and password, the sign-in and updated values will update together. Additionally, if you are using the Remember me feature to keep your session alive across multiple days, you may also see periodic updates to these values when you access your account coinciding with additional verifications of your session. In both of these login scenarios the Last sign-in value on that user is updating, so the Updated value changes to that same time to signify a property of that user had changed.