Another factor is homogeneity of the offspring: parent lines are very much homozygous. If one parent line is AA at one locus, and the other line is aa at the same locus, the children will all have Aa. Meaning they have both versions of the gene (which is usually good, especially in immunity and health) and all have the same status. This means you can produce a couple of hundred piglets grow to the same size in the same time which makes processing and marketing them easier and cheaper.
Another factor is that some breeding goals are mutually exclusive. Piglets for example drink milk from the mother, which means she must have good milk production and other fertility related features, whereas the father can be bred more for muscle growth.
And then there are other weird tricks breeders can pull. I believe in chicken there are experiments with hybrids that hatch in different colors relating to their sex, or don't hatch at all if they are the "wrong" sex.
Another factor is homogeneity of the offspring: parent lines are very much homozygous. If one parent line is AA at one locus, and the other line is aa at the same locus, the children will all have Aa. Meaning they have both versions of the gene (which is usually good, especially in immunity and health) and all have the same status. This means you can produce a couple of hundred piglets grow to the same size in the same time which makes processing and marketing them easier and cheaper.
Another factor is that some breeding goals are mutually exclusive. Piglets for example drink milk from the mother, which means she must have good milk production and other fertility related features, whereas the father can be bred more for muscle growth.
And then there are other weird tricks breeders can pull. I believe in chicken there are experiments with hybrids that hatch in different colors relating to their sex, or don't hatch at all if they are the "wrong" sex.