You are already getting the pan hot enough to sear, just leave it there a little while longer and it’s cooked!
For a thin steak, I agree. However, a thick cut of meat requires a longer cook time and the crust applied at the beginning of the cook tends to lose its integrity. Cooking longer and slower first allows one to apply the desired temp evenly throughout a thick cut, then searing last reduces and even eliminates the tough grey outer layer.