This is definitely not because prehistoric people were unable to get clay of greater purity. On the contrary, the clay they used was often of higher quality than clay today.
這並不是他們無法取得好的陶土，相反的，史前時代，地質經水解、沈積作用，形成層狀、均勻度很高的泥質，陶土的品質比現在所能取得的更好。