Time. The very first homo sapiens are estimated to have diverged from other hominids around 300,000 years ago. The first city that we're aware of is Uruk, which was in the Mesopotamia area, and was likely founded somewhere around 6,500 years ago. This means that for just shy of 98% of the existence of homo sapiens, there were no cities. Some degree of settlement did exist before Uruk, however. The earliest agricultural evidence I'm familiar with is from around 15,000 BCE, or 17,000 years ago (in the Levant, modern day Syria/Israel).

Urbanization has been a massive driver for human population. Every major innovation that has increased our ability to build sprawling cities has massively increased the human population. When the Pyramids of Giza were built (around 2,600 BCE), the world population was somewhere around 20,000,000. 3,000 years later, which represents less than 1% of the total existence of humanity, in 600 CE, the population of the world was closer to 200M, 10 times that. We hit a billion just 1,200 years later, in 1804. Now, 120 years after that, we're approaching 7.8 billion.

So, to summarize, its simply that with the way populations have moved over the last 300,000 years, and the way the growth of human population has increased exponentially over the last ~1,000 years or so, when nearly all areas of the globe had been settled for millennia.