Humans also die, and so have to give birth to new people and train them over and over again to maintain and develop social and economical structures. Artificial brains don't have to die, they can be also cheaply duplicated, and their old versions can be stored and used later if need be.

humans usually die after several decades. They do take decades to train. It would be beneficial when we live to 110 and work to 100. At any rate, we don’t do enough with our existing time.

When we focus time and effort on things like the Manhattan Project or the Apollo Program, we are able to accomplish a lot in a short time.