Until recently, Kali Linux used the root account by default. In the latest versions of Kali, root login is disabled, forcing you to login to the GUI as your normal user account. The reasoning behind this change should be obvious, but if you're a Kali user, you probably know your way around a Linux system by now, and there's not much risk in letting you use the root account to login. Kali developers must've known that this change would annoy some people, because they've made it very convenient to re-enable the root login.
This is a companion discussion topic for the original entry at https://linuxconfig.org/how-to-enable-root-login-on-kali-linux