(not an unix sysadmin, just guessing what happened from my shaky knowledge)
cron jobs reports activity by email to the user (UID) they are running, historically UNIX boxes have the ability to handle mail locally (people would leave messages to each other by connecting to the same server via terminal), so that the root cron activity would land into the root (/root) account mbox file.
When email got interconnected more across servers, generally the service that would dispatch mail to the users account on their home folder on the server started to be able to forward to to others servers, if a domain name was provided. Add to it the ability to fallback to a _default_ domain name for sending email into the organization, and voilà, the root email account for the default domain name receives the entirety of the cron jobs running under root of all the servers running with the default configuration and domain fallback.
cron jobs reports activity by email to the user (UID) they are running, historically UNIX boxes have the ability to handle mail locally (people would leave messages to each other by connecting to the same server via terminal), so that the root cron activity would land into the root (/root) account mbox file.
When email got interconnected more across servers, generally the service that would dispatch mail to the users account on their home folder on the server started to be able to forward to to others servers, if a domain name was provided. Add to it the ability to fallback to a _default_ domain name for sending email into the organization, and voilà, the root email account for the default domain name receives the entirety of the cron jobs running under root of all the servers running with the default configuration and domain fallback.