Here's why:

* The Human Genome Project: This project mapped the entire human genome, revealing that we have about 20,000-25,000 genes.

* Gene Expression: Not all genes are active at the same time. Gene expression is the process of turning genes "on" or "off" based on the needs of the cell and the organism.

* Cellular Specialization: Different cells in the body perform different functions. This means they only need to express the genes necessary for their specific roles. For example, muscle cells express different genes than brain cells.

Therefore, only a small percentage of our genes are active at any given time, typically estimated to be around 1-2%.