> Why do you still need sudo permissions to install anything on most Linux systems?

You don't with Flatpak or rootless containers, that's partially why they're being pushed so much.

They don't rely on setuid for it either

Flatpak and AppImage.

Or download & compile & install to a PREFIX (e.g. ~/.local/pkg/), and use a symlink-manager to install to e.g. ~/local (and set MANPATH accordingly, too). Make sure PATH contains ~/.local/bin, etc. It does not work with Electron apps though. I do "alias foo="cd ... && ./foo".