To put it bluntly, if you're taking your kid out to teach him an academic curriculum that directly contradicts the actual school curriculum, why should that kid be awarded any kind of graduation diploma that a good college would more-than-sneer at?
Just to take a sample of people posting in this thread, I know at least two are young, homeschooled kids iirc... and whaddaya know, both of them think evolution is crap. What a coincidence.
Actually, I have yet to meet someone who was homeschooled through HS and didn't turn into a Young Earth Creationist, so there you go. I have a feeling that an enormous percentage of these kids are basically victims who were taken out of the real schools by their crazy parents who didn't want their kids "indoctrinated" by the ebil "secular humanists."