Jeeesus Checkboard! I think I might just give my engineer licence back to the authorities, I am obviously not worthy!
As you said, complex answers for a simple question. As the aircraft frame is encountering less air resistance at higher altitude, there is less requirement for power to propell it at constant speed through the air. Therefore, as the air is less dense, the optimum fuel/air ratio is maintained by reducing the fuel input. Modern turbofans are gas path optimised for best SFC in CRZ also. The advantage they have over a piston engine at high altitude is the high mass airflow the turbine engine design can process, whereas the piston engine mass flow is a limit of the swept volume, and the ability of other equipment to compress intake air into the cylinder at pressures that allow the engine to still function. Remember, the turbine engine mass airflow capacity is variable, whereas this is not exactly so for the piston design.