Why do we do Hypothesis Testing?

Question

Generally a stat book is divided into two parts: Hypothesis Testing and Estimator Theory. Given data points, in hypothesis testing, we choose between null $H_0$ and alternate hypothesis $H_1$ for certain entity (for eg mean). My opinion is that why can't I choose an estimator from data and directly evaluate it. This way I will get the same answer? (Infact it will save me the trouble of even guessing the $H_0$ and $H_1$.