Isn't it the temperature needed for enzymes to properly work their metabolic magic (1) temperature helps reaction rates 2) enzymes need a certain temperature to maintain their shapes)
It affects peak efficiency of enzymatic reactions, but that's a tradeoff because now the species has to maintain internal temperature within a narrow band or they die. A common alternative is to allow more fluctuation of core temperature at the cost of a lower overall efficiency. That's basically mammals and birds versus reptiles and fishes.
But enzyme production mechanisms are subject to evolution too, no?
But also to basic thermodynamic considerations. As far as chemical systems go, mammals actually operates near the low end of temperatures.