Precast concrete tends to be more durable than cast-in-place concrete. The carefully monitored curing process of precast concrete sets the seal on the quality of the product's finish.  Another factor which lends itself to precast's supremacy is the lack of disruption from external factors such as weather conditions. With in-situ concrete, you have to be wary of rain, hail and other elements which may serve to harm the final form of the casting.