First all developers are extremely talent and some bagged years of production experience under belt before they formed ID.
Second as other mentioned these are relatively simply games, not the ones like Ultima and Golden box that usually takes years to develop.
Third I think there is a unique culture in teams such as early ID: No bullshit, devs willing to devote 80, 100 hours per week, no management overhead as they KNOW what they are going to do.