You know how "the average person is really dumb, and half are worse"? Well, it's not that people in general are especially stupid, nor that some are exceptionally smart. The key is that most people don't actually think much most of the time, while some do.

Overall, the level of "raw" intelligence that people have doesn't really differ by a whole lot. But the difference between two regular people, one who thought something through and one who didn't, is in fact very significant. That's why people say "think before you act", because simply taking the time and thinking it through is usually enough - while not doing so could lead to disaster.

Now consider a person who often takes the time to think about things - every day their understanding of the world increases. If they also read and seek knowledge from others, even better. But a person who rarely does this will barely grow at all.

Over time it adds up, and then whenever something complicated comes along the regular thinker doesn't need to start thinking from scratch, they've already got a head start because they've thought about some of the relevant things before.

When you're able to quickly figure things out, because of previous thinking, then you'll enjoy it more - just like being good at anything else - and surprise, surprise, you'll find yourself thinking about things more often. A self-reinforcing behavior leading to a vast gulf between people who act intelligently and those who don't.