Why does the fact that Green House Gases are a very low percentage of the atmosphere matter to people?

Why do alarmists always compare green house gases with something like botulism, hormones, or cancer? They aren't. They moderate our climate but alarmists claim that they cause increased warming and cooling but they never let facts get in their way. Pretending that CO2 is harmful and that small changes in the concentration of CO2 is driving our climate is not scientific. Comparing it to cancer is way over the top.