Why does Debian-Ubuntu not provide a simple command for this?
You aren't supposed to add repos. Ever. https://wiki.debian.org/UntrustedDebs
Apt is not built with security in mind, at all. The partial sandboxing it does do is trivial to bypass. Adding a repo is basically a RAT Trojan on your computer.
An example is signal-desktop
Yeah don't use signal. They restrict freedom 3 by making distribution difficult. Thats why they trick you into using their RAT repo.
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=842943
The least bad option is the unofficial flatpak.