Great question. Unfortunately, we don't know. Either the homozygous Spider is lethal or it simply doesn't exist. It is different than breeding other genes because, well, it is another gene.
For a little more clarity, Spider x Spider pairings themselves do not lead directly to this result. It's only when a copy of the Spider gene gets passed on from each parent that we run into this. It is perfectly normal to hatch a clutch of otherwise healthy ball pythons from two animals with the Spider gene. These pairings are just not as common due to the feelings regarding the homozygous Spider and the accompanying morality concerns surrounding this issue.
For more information on this, and many other morph-related concerns, here's the OWAL Reptiles - Issues page: http://owalreptiles.com/issues.php.
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