Here's why:

* Gene expression is regulated: Cells carefully control which genes are turned on (expressed) and which are turned off. This allows cells to specialize and perform specific functions.

* Cellular context: The genes used by a cell depend on its type, its location in the body, and the signals it receives from its environment. For example, muscle cells express genes for proteins involved in contraction, while nerve cells express genes for proteins involved in transmitting signals.

* Temporal regulation: Gene expression can also vary over time. Certain genes are only expressed during specific developmental stages or in response to specific stimuli.

Think of it like this: Imagine a library with thousands of books. You might only need to read a few books at any given time, depending on your interests and what you're studying. Similarly, cells only use a subset of their genes at any given moment.

Examples:

* Blood cells: Only express genes for hemoglobin production, a protein essential for carrying oxygen.

* Immune cells: Only express genes for immune system components when needed, like fighting off infections.

Therefore, the vast majority of genes in a cell are not being used at any particular time. This controlled expression is essential for maintaining cell function, development, and responding to environmental cues.