When going through Activity Monitor, I see many different users (other than my own account and the root account) responsible for individual processes. Why are the processes divided among many users that are created by the operating system? Why are they not simply divided amongst the root account and the user account currently in use?
This is the principle of least privilege.
The principle means giving a user account or process only those privileges which are essential to perform its intended function. For example, a user account for the sole purpose of creating backups does not need to install software: hence, it has rights only to run backup and backup-related applications. Any other privileges, such as installing new software, are blocked.
Many daemons only need access to a specific hardware device or to specific files, so they run under a dedicated user account. This is done for security: that way, even if there's a bug or misconfiguration in one of these services, it can't lead to a full system attack, because the attacker will be limited to what this service can do and won't be able to overwrite files, spy on processes, etc.