It is not at all about how reflective mirrors are, but how they reflect.
Mirror Reflection: 
This occurs when the incident ray hits a smooth or glossy surface such as glass. The only time light will be reflected directly back to source is when the incident light is directly at right angles with the reflective surface. All other times the light will be returned in the opposite direction. That is called specular reflection. No matter how highly reflective the material may be it makes for a terrible reflective material for plant growing.
Diffuse Reflection: 
This occurs when light strikes rough or uneven surfaces, these types of surface cause the rays of light to be scattered in many directions. That is what you want for growing.