I can verify this -- as an acknowledged "computer person" and "rational person", I still didn't back up my data, even while advising my friends that they should and they'll be sorry when they don't. Fortunately, my hard drive started making interesting new noises, rather than failing without warning, so I didn't embarrass my self too badly. It is fairly common for someone to acknowledge and advise others of backing up their data, but failing to do so themselves.

I think it's a combination of procrastination, laziness, being super-cheap, optimism/arrogance, and not having especially valuable data. Though people with valuable data do it too.