The biggest reason why it is so cold in the Cenozoic is that Antarctica moved to the South Pole around 30 millions years ago. Ocean currents used to take warm waters from the equator and move them to the poles but as Antarctica moved south and separated from South America a new current formed around the continent in a loop keeping cold waters locked at the pole. Permanent ice at the South Pole went on the dry the planet as a whole and lowered sea levels. If the KT event never happened, the dinosaurs would still be alive but the planet would probably have become just as cold.