* Generational overlap. Each generation is shorter than the one before it, so at any given time there are more people alive in the older generations than in the younger generations. This means that you are more likely to have multiple ancestors in the older generations than in the younger generations.
* Population growth. The human population has been growing steadily for thousands of years. This means that there are simply more people alive today than there were in the past, so you are more likely to have ancestors in the past who were related to each other.
* Migration. People have been migrating from one place to another for thousands of years. This means that your ancestors may have come from different parts of the world, and you may have ancestors who were related to each other even though they lived in different places.
* Intermarriage. People have been intermarrying with each other for thousands of years. This means that your ancestors may have married their cousins or other relatives, which can lead to multiple lines of descent from the same ancestor.
As you go back further in time, the likelihood of these factors occurring increases, so you are more likely to have many of the same ancestors further back you go.