[Fanta]

Because of levering effect.
With spacer, longer bolt or bolt conversion which will change physic calculation, such as the longer the level will be easier to lift up an object; provide more force to lift up an object.
Under this situation, wheel bearings will be premature damage. Wheel bolt will under more level forces and spaces between wheel and hub will increase. Although you are using a very thin spacer, car is a very heavy object and traveling very fast, an exponents calculation.