In one sense your question is similar to many others: Why should the sky be blue? Why should gravity at the surface of the earth accelerate objects downward at 9.8 m/s2? Of course, the answer to question of economic growth is more changeable, dependent on societal arrangements that are more likely to change than physical conditions governed by natural laws for the other two questions. But given the economic society we have, 4% or so is the observed stable growth, worldwide.