Because gods are an exception to the rule that humans aren’t completely stupid. Wait, let me explain that!
- If I tell you I can bench press a Hummer, you won’t believe me unless you can see me do it.
- If I tell you I have a 20-inch penis, you won’t believe me until I drop my pants.
- If I tell you that I can double your investment within one month, then unless greed completely shuts down your brain you won’t believe me.
This is how it works for most things. Sensible people will reject extraordinary claims until they see evidence. That even applies to a lot of people who aren’t very bright. It’s normal human skepticism.
Gods make an end run around the human brain’s scam protection by cheating: They get help from your parents. Did you notice that almost everybody has the religion of their parents? The reason is that belief in gods is (almost always) transmitted to children by their parents, who got it from their parents. The reason is that children under a certain age, maybe around 10, haven’t yet developed that capability for critical thinking, for skepticism, that keeps people from buying the Brooklyn Bridge.
Religious people think that ancient books like the Bible, the Koran or the Bhagavad Gita are evidence of their particular god-fantasies. That’s ridiculous! An objective reading of those books shows that not only are they full of un-evidenced claims about magic and impossible events, they’re even also full of self-contradictions and frequently make claims at odds with modern science, which tends to get stuff right. The fact of the matter is, those books only make sense to people who are already victims of the swindle because – see above.
An atheist is someone who either
- was never persuaded by his parents to believe in some god(s) or
- managed to see through all that monkey business and realize that god-beliefs make no sense because they’re completely unsubstantiated.
But you’re asking “how can atheists be so sure?”
First, realize that you’re misinformed. Not all atheists are sure there are no gods.
- Some don’t believe there are any gods but consider it possible there are one or more gods we simply haven’t seen yet. These folks are still sensible enough to not believe in gods until one is actually sighted. Philosophically, these people are referred to as agnostic, or agnostic atheists.
- Others don’t believe, and are somewhere between “sure enough” and “absolutely sure” that there are no gods, simply because all the human god stories are so ridiculous that it stands to reason they were all made up by uneducated, possibly nutty humans.
- Presumably, there’s also a “middle” category of atheists who are very sure that none of the human-invented gods exist (because what people say about them makes so little sense) and thus all theistic religions are false, but that there could be real gods that no one’s ever encountered or described.