Uhh... Because it puts your young children at risk when they're not old enough for certain vaccinations yet.... It's not rocket science... And that's not even getting into the fact that no vaccines are 100%.... The slightest bit of research would tell you that. Just like with condoms not being 100%, everything has a fail rate and I would rather not gamble mine on whether or not the mom sitting beside me had her child vaccinated against measles and more.