Real Life propellers also have adjustable pitch and and real life engines has fuel/air mixture.  All of this is abstracted on KSP. Performance props adjusts the throttle in order to keep the engine on the best fuel/power rated RPM. The props are then pitched to get the needed thrust given that RPM. The fuel/air mixture lever is adjusted so you don't inject more fuel than is possible to burn given the current altitude' air pressure and oxygen availability, and so avoid wasting fuel by throwi