Paint by Numbers

Mamet is blowing my mind. So in the chapter Paint by Numbers we're are given the wisdom of rehearsal. Now I suppose that if you have a set of competent actors then rehearsal would not have to take long. However with out training how does one get a competent actor? Mamet says himself that an actor needs to be trained to speak well etc. But with out school how do you get that. Even if you get experience on the stage it means at some point in time you will be in a show that is not full of competent actors and there for rehearsal just can't work that way. See blowing my mind. It feels to me like he is over simplifying and looking at an unattainable ideal.