Physics and genetics are two different things. It could be that homozygous spiders and pins are lethal and either people experience egg death or follicle absorption as a result (which has not been proven either). We won't know until we can prove it out. Is there a homozygous spider or pinstripe out there? Maybe.....but we cannot say for certain that there is. We cannot assume there is just because a punnet square tells us it is so.

Years ago, people devoted to science told us the earth was the center of the solar system because the stars and planets seemed to orbit around us in the night sky. And for the longest time, this was the truth and the only way of thinking about how the universe was oriented. Then science changed and they proved we in fact revolve around the sun. So there is plenty we still do not know about genetics. The spider and pinstripe gene might be one of those things we just have not figured out yet. Maybe you have to combine the spider gene with another color/pattern mutation in order to unlock the homozygous potential. Perhaps the spider gene simply requires a new mutation that has not been discovered yet. One that would allow there to be a homozygous animal. Maybe it is sex-linked. Perhaps the spider gene isn't a dominant gene but in fact in a whole different ball park of genetics and we only say its dominant to make it easier to understand. We won't know until someone proves it out. So far, no one has.