Because Windows is an example of what happens when people build an OS around the "usability"-trumps-security attitude you espouse. I was not entirely serious when I said to switch to Windows, but it provides a perfect example of what happens when people think the way you do about security. The OS is so horribly insecure that within mere minutes of first connecting to the Net, your average machine is already hacked.

Not that it matters. What you are advocating is not usability, but convenience, and you're advocating it at the cost of necessary security. You are the one who does not "get it".