Why is the fossil record an incomplete history of life?
Imagine you have a jigsaw puzzle. Scatter it all over the surface of the planet. Now wait a million or so years and then start looking for it. Say there are 400 pieces and you've found 25. How well do you think you can figure out what the puzzle is?
Because we have not discovered all there is yet. Be patient.
Besides the obvious fact that we haven't dug up every square inch of the earth, fossils are hard to form. It's pretty amazing we have any.