Guest post by "Gunsolo" The major arcana are made from a mixture of different families from the minor arcana, right? So perhaps then they are infertile in a similar way to mules?
"Mules and hinnies have 63 chromosomes, a mixture of the horse's 64 and the donkey's 62. The different structure and number usually prevents the chromosomes from pairing up properly and creating successful embryos, rendering most mules infertile. There are no recorded cases of fertile mule stallions." -Wikipedia