[Big time]

A. Is it because MB has better design tools like wind tunnels, superhigh speed computers for CFD, top aerodynamicists, etc?

or
B. MB can afford to produce more complex body shapes. Maybe aero shapes can't be formed in the so thin sheetmetal usually found in Japanese cars? Even if aero shapes can actually be made in thin sheetmetal they require more complex molds and robots that only MB has?

You can even add
C. MB rear wheel drive allows for a lower hood line. Maybe also better internal aerodynamics after air passes thru the radiator? Although modern MBs have a quite crowded underhood space.