well actually the material that vehicles are made of does play a role in ride quality. The chassis stiffness both beam and torsional rigidity plays a role which is a function of geometry and material properties. However, vehicles have suspension and the % influence on ride quality from chassis material is minor.

On a bicycle, there is no suspension, thus the chassis(frame) stiffness will be a much larger portion of the overall ride quality.

did you really need me to explain that for you?