Because an internal combustion engine uses heat to push a piston down. More heat equals more push.
That's why modern cars run around 200F and don't have cold air kits (unless they're a special car that's marketed with ''race car'' stuff on them.)
Max efficiency at road speed vs max power production has to be separated.
If interested you can go lookup/figure out the heating value per unit gasoline and the equivalent of what is put out to the wheels, take the difference and figure the efficiency of the engine. You can do this for various operating points, and determine what are the driving factors that determine max HP and max efficiency. Max HP is going to be dependent on maximum mass flow through the engine. You don’t want to blow it up, so it has to be a sustainable process. Max efficiency is extracting every last BTU from the fuel to maintain road speed.
In order to determine the effect of inlet air temperature on power production, the additional mass flow and so the additional fuel and heating would have to be considered against the energy to take the charge from 90F to say 1500F(arbitrary guess) vs taking the charge from 150F to 1560F. Just remember the 90F charge has more oxygen and fuel than the 150F charge. I’m sure that is over simplified, but it paints the concept, because the 90F charge has more fuel, peak temp may match or exceed the what 150F charge atains. This is not taking into account any combustion dynamics, of which I don’t know enough to talk about. Other than to say hotter charge temps seem to be more prone to knock which kills power and/or engines.
The car companies want higher engine temps and charge temps for efficiency, not peak power. Generally the combustion process is considered adiabatic, but you do have heat loss. The heat loss when looking at the max power case is probably not significant, but when looking at a car that maybe needs 20 to 25 HP to maintain road speed, heat loss will play a role. Being able to increase operating temps, decreases the delta T between the combustion chamber and the coolant, which will retain more heat to drive the engine and increase efficiency at road speed. Very little benefit at the strip though.
