Why is everyone so quick to ume that global warming exists?

I agree with you. Global warming likely doesn't exist, but climate change does. You are indeed correct about earth's history and indeed there is little evidence that directly links us to climate change. The temperature dropped during the post-war boom due to increased cloud cover. This was just cause back then while we were emitting more CO2 we were also emitting more sulfates cause the fossil fuels were were burning weren't as clean as today's (we hadn't figured out acid rain etc yet). The increased sulfates acted as nucleating agents for atmospheric moisture and more clouds formed which reflected more solar radiation and hence the temperature drop. Even looking at climate records for the past 3 million years the global temperature is decreasing. In the cretaceous CO2 levels were 3 times what they are today and were certainly weren't around. So as was said earlier, climate is going to change with or without us, and we have little or no say in it.