Most distributions encourage their users to use normal user privileges while running the operating system. This is sound security advice, as this behaviour provides an extra layer of security between the user and OS. This is especially true for multiple user systems, where user privilege separation is required.
By nature, Kali Linux is a security and auditing platform, where many tools need to run with root privileges. Generally, when using Kali Linux, being in a multi-user environment is unlikely and therefore the default Kali user is “root”. Additionally, Kali Linux is not recommended for use by Linux beginners who might be more prone to making destructive mistakes while using the super user account.