Why would mating between a horse and human never result in centaur?

Mating between a horse and a human would never result in a centaur because it is biologically impossible.

Here's why:

* Species are too different: Horses and humans are different species, with vastly different genetic makeup. They are too distantly related for their DNA to be compatible enough to produce offspring.

* Chromosomes: Horses have 64 chromosomes, while humans have 46. For successful fertilization and development, the chromosomes need to pair up correctly. The drastic difference in chromosome number makes this impossible.

* Developmental incompatibility: Even if fertilization somehow occurred, the resulting embryo would be unable to develop properly due to the vast biological differences. The development of the embryo is controlled by a complex interplay of genes, and these differences are too large to be overcome.

In short, the idea of a centaur is purely mythical and not grounded in any biological reality.