Pardon me, but isn't protection against security breaches the OPERATING SYSTEM'S JOB???
Partially, but it isn't the operating system's job to stop the user from being an idiot. If you want to run executables from suspicious websites, that's your right. And if the rest of the world wants a device to stab you in the face over the internet, that's their right, too.
I have two browser windows open at the moment, but they have probably 200 tabs between them - most of them from... well, take a guess.
Even if we cut out the TV Tropes tabs, I'm still probably in the 20 or 30 region.
All constants are variables.