It also says "community curated". The books are crowdsourced. Most contributors are clearly what you'd refer to as hackers, but there's nothing stopping you from posting a Philosophy book. I think it mostly points to the fact that (I'm willing to bet) most authors who'd willingly make their books available free of charge are of a technical bent. There are philosophy, economics, psychology books on the site. If you find any and you've got some time to spare, you're welcome to add to the site.