Since space is mostly empty space, the process of losing heat is very slow. Typically heat transfer happens through conduction and convection, that is a warmer object is bleeding off heat to surrounding matter, which is typically a gas or fluid surrounding it. In space, this effectively doesn’t happen, since there’s very little matter in between bodies to transfer that heat to. So most of the heat is transferred through infrared radiation. This is a much slower process than the other two, and in the case of planets their cooled outer shell acts as further insulation which means that the rate at which a planetary body cools gets slower as the outside is cooling. In the case of Earth there’s even more insulation because of the atmosphere.
So the Earth is cooling, just very very slowly.
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