>I'm not understanding how it's the desktop Linux users who have to deal with poor security.

On Linux Mint if you run a program without granting any extra permissions it can: Record your mic, record your camera, record your screen, steal your browser history/ cookies/passwords, alias sudo or show a fake update dialog to collect the user's password to elevate to root, see if you copied a crypto address and replace it with a similar looking one owned by the attacker, encrypt all of your files, send any sensitive pictures or documents to the attacker, etc.

The existence of a 50 year old concept of file permission is not good enough to combat the modern security problems users can encounter.