> Desktop OSs have been able to do these sorts of things since the 1980s without all the hoops Linux Desktop software makes you jump through to do them.
Linux desktops can do this. Flatpak maybe can't (Idk, I don't use them that often, but I'd be surprised if they couldn't), but even before AppImage, Flatpak, Snap, etc you could always just run an application from wherever you want. You could do that 20 years ago, and you can do it today.
Desktops in the 80s didn't have sandboxing, but modern Linux desktops do. You can either implement sandboxing yourself through syscalls, use something like Systemd, or use the significantly more user friendly Flatpaks.
...Or don't use sandboxing at all, and do whatever you want. Download a (hopefully statically-linked) ELF from the internet, chmod +x it, then execute.
> standard in Linux Desktop application distribution
There is no such thing. There never has been, and there never will be :P