Because having one engine that drives all 4 wheels is a terrible design that adds complexity and weight, but is the only realistic architecture with ICE vehicles.
The best design would be an electric motor inboard of each wheel.
There were some ICE cars that used two engines to achieve AWD also, but in a practical sense it's hard to have two engines (especially one front and one back) in a ICE car. Takes up all your cargo space, plus you have to deal with two emission systems.Stupid question, but you know how for gas AWD cars, they only have 1 engine; so technically Tesla can implement AWD using just 1 motor, right? Why do they use 2?
Imagine the insane levels of grip and maneuverability that 4 independent controlled motors could bring, especially with world class dynamic stability control.