Ha, I find it funny that you consider this to be "such a dirty keyboard", when doing a google search for "dirty keyboard" turns up results that are apocalyptica

Ha, I find it funny that you consider this to be "such a dirty keyboard", when doing a google search for "dirty keyboard" turns up results that are apocalyptically dirty in comparison to the keyboard in this article. This one isn't spotless, but "wow that keyboard is dirty" wasn't even close to what I was thinking when I saw that picture. To answer your question more directly: my MBP keyboard is usually spotless due to my obsession with cleaning it, but even just spending half an hour working outside on a windy day, or using my laptop while at my parents house (who have two large dogs that shed a lot) will result in it looking like that. And to the article's point: such scenarios have lead to some very annoying keyboard issues with my MBP.