In the early 1980s the first affordable ``household'' computers such as the Vic 20 and later the Apple II hit the market, and digital computers entered the collective consciousness. This was also the time of the first BBS systems, acoustic couplers and movies like Wargames where a young nerd finds a backdoor in a Pentagon system that allows him to talk to their nifty new computer controlling the entire U.S. defense. Which takes the world (1983 seems to be so far away) to the brink of nuclear war, unintentionally of course.
In Wargames, computers are huge thingies with lots of blinkenlights that can think, and talk in synthesized voices (clearly, voice distortion was synonymous with high tech at the time). The movie is similarly spohisticated about the security aspects of computers. Dialing the right number and guessing the correct password is all it takes the bespectacled computers wiz kid to break into the Pentagon's most sensitive computer.
I'm not sure if this movie has anything to do with it, but until quite recently, most people thought of computer ``hackers´´ as socially unbalanced adolescent greasy-haired nerds spending long, lonely nights in front of their computer, consuming fast food and trying to guess passwords. Given this threat perception it is obvious that many network administrators felt protected by the laws of statistics. There can be only so many socially unbalanced etc nerds, and since password guessing is such an inefficient and cumbersome technique, the probability of them breaking into my computer is so small it's not worth losing sleep over it. QED.
If you've followed computer security mailing lists for a while, you will know that said socially unbalanced etc nerds have also noticed that guessing passwords is quite inefficient, and started looking for better ways into other people's systems. And unfortunately, they have become very successful and very efficient at what they do.
To be precise, I'm not even sure the socially unbalanced nerd image is right. Many security professionals still like to think of their adversaries as script kiddies who pick up shrink-wrapped sophisticated attack tools and use them without much clue. And while there definitely are some aspects to the cracker scene that will unfailingly remind you of bragging 15 year old boys, there are surely many very bright and clueful people who know their stuff, and who should not be underestimated.
XXX: Robert Redford etc in Sneakers!
And don't be so sure statistics will protect you! Today's cyber vandals routinely use mass scanning tools to check huge numbers of hosts for certain vulnerabilities. Your site's information may not be useful to them, but they may still want to use your host as an intermediate hop when attacking their real target, if only to conceal their tracks better. Or they make your site part of a distributed denial of service attack like the ones that took down Amazon.com and others in March 2000 by flooding it with network requests. Or they break into your machine out of plain curiosity, and in order to hide their traces they format your hard disk.