What you call common sense is really just the actions of the educated and informed being so ingrained that they feel natural. An internet forum's user-base is de facto better informed than the average computer user - what seems obvious to us is like a foreign language to many. And it's not just kids and grandmothers - there are plenty of intelligent adults who spend their whole workday using computers, yet absorb absolutely nothing about how they work. Those people have to have their hands held at all times - training won't do any good. You best hope is to contain them, and try to minimize the damage they can do to themselves. Ergo, Windows and antivirus shells.
9. There is so much money now in computer security, and so many companies making a profit, that it gives these security companies a self-interest in CREATING viruses and making people be insecure and feel insecure, without their security "solution".