Because science isn’t always science, sometimes its just another religion with rigid dogma.
During the Cretaceous Period 145 to 66 million years ago polar regions were free of continental ice sheets, their land instead covered by forest. Dinosaurs roamed Antarctica, even with its long winter night. I have yet to read a reasonable explanation for this phenomenon. I personally think the sun was putting out more energy during that era.
Analysis of bubbles trapped in late Cretaceous amber show that the atmosphere contained around 2,300 ppm of CO2 and 30% O2 by volume. The sea level air pressure was higher.
This accounts for how dinosaurs and pterosaurs reached sizes impossible in today's atmosphere. A lot of land was submerged, and the greater ocean coverage and elevated CO2 levels contributed to the uniform global temperature. Despite the very high CO2 concentration, the oceans didn't become acidic (contrary to today's eco-hysterics) but instead formed extensive chalk deposits and luxuriant coral reefs.
6000 ppm Co2 was a thing at a prior time. That’s about fifteen times the current value. The Co2 thermal augmentation effect is tied to a logarithmic progression, so most of the heating happens with low percentage values of the gas in the mix. Water vapor is the real driver of heat retention.
Ocean circulation was different such that more warm surface flows similar to the Gulf Stream, carried oceanic warmth to the polar regions. The formation of the Isthmus of Panama made a large contribution to cooling.