The fossil record is incomplete for a few reasons:
Fossilization is rare
Fossilization is a rare event that requires specific conditions, such as rapid burial and the absence of oxygen.
Fossils are difficult to find
Fossils are buried underground and typically must be dug out.
Many fossils have been destroyed
Many fossils have been destroyed by geological processes, such as erosion and plate tectonics.
Soft-bodied organisms are underrepresented
Soft-bodied or cellular organisms such as invertebrates or prokaryotes are the most unlikely to be fossilized and underrepresented in the fossil record.
The fossil record is still very important because fossils allow us to get a glimpse of the organisms that lived throughout history.