because that isn’t really how these things work. It doesn’t matter how secure your operating system is, it can still get infected with malware if you let the user download and install arbitrary software. every modern desktop operating system that allows this has this hole.

features that pop up warnings when running code not signed by the OS vendor (like Windows SmartScreen or macOS Gatekeeper) help to an extent, but are not magic bullets since users can still override them.

at the end of the day, the best defense is to make sure you actually trust any software you download before running it.