Clinical trials are necessary because regardless of the amount of pre-clinical trials you conduct, nothing done pre-clinically will directly transfer over to a clinical observation. In other words, using something similar to humans is not the same as using humans. For example, if you are testing the fibers of the sheep spinal discs because they are similar to human discs you would still need to test human discs. This is because sheep are quadrupeds and their discs impact differently than our bipedal discs.