For example, suppose I want to prove that "All Crows are Black."
Next, a thought experiment. Suppose tomorrow I inspect every single crow around the world and I find that indeed every crow was in fact black.
Question: Have I proved that "All Crows are Black."
Answer: No, I have not. The reason is because I did not inspect all crows that existed previously and all crows that would exist in the future, which I cannot do. In short, I do not have the "entire population."
Thus, the need for Hypothesis testing, which is essentially based on what is referred to in logic as Modus tollens i.e. proof by contradiction.