Question

Why must a scientific hypothesis be testable?

Answer 1

An hypothesis need to be testable to clarify the prediction you claim within the hypothesis, and if this hypothesis came with a lot of evidence and the evidences match up with the hypothesis, eventually will be a theory

Answer 2

The whole point of science is that it the ideas it produces can be either verified or proved false. Those ideas begin as hypotheses, and if hypotheses can't be tested, then there's no way to either verify or falsify them. Without this kind of empirical process (in which we use evidence to confirm or deny a fact), then hypotheses no longer belong to science, but become something like philosophy.