Why must a scientific begin by making observations?

Generally, one makes a casual observation - Apples fall from a tree - and from this one generates an hypothesis - a casual explanation for the observation.

With the hypothesis in mind, an experiment is designed to test or prove/disprove the hypothesis.

With a sufficient number of experiments under your belt, you may now form a theory, which is a much firmer type of being. It may be capable of more general application.