Same can be said about loopback device in general. Why do you need to use networking when you are literally on the same device and can use binder/dbus and friends with native apps?

Shizuku uses Binder AFAICS[^1]. Looking deeper it seems that Shizuku does not connect to the device itself per se, but rather it has a privileged server launched manually through adb. Never used Shizuku, so can't say for sure.

[1]: https://github.com/rikkaapps/shizuku#how-does-shizuku-work