You can’t spawn actors in the construction script because it runs when an instance of that class is created or modified in the editor. So it runs when you place an actor in the level, when you move it or modify any other parameter on it and it would be quite unfortunate if it started spewing actors all over the place. 
It is helpful to think about the construction script as the place to get the actor ready internally for the game and for the editor (should the level designers need to work with them). That is the place to create/modify components and to calculate/store data to be used when the game begins.