Here's why:

* Fossil formation is rare: For an organism to become a fossil, it needs to be buried quickly in sediment, protected from decomposition, and subjected to the right conditions for mineralization. This is a very specific set of circumstances.

* The vast majority of species have gone extinct: Throughout Earth's history, there have been five major mass extinction events, along with countless smaller extinction events. This means the vast majority of species that have ever lived are no longer around.

* Fossils are biased towards certain types of organisms: We are more likely to find fossils of organisms with hard parts (like shells, bones, or teeth) that are more easily preserved. Soft-bodied organisms are rarely fossilized.

Therefore, the fossils we find are a tiny snapshot of the incredible diversity of life that has existed on Earth. The fact that most of the species represented by fossils are extinct is a testament to the constant process of evolution and the ever-changing nature of life on our planet.