Why the material used to build the reedblock has little or no effect on the sound?

https://accordionguide.info The reedblock is the piece of wood on which the reeds (plates) are mounted.
The cavity under the plate and inside the reedblock is called:

THE TONE CHAMBER

The tone chamber is important for the accordion tone's fine adjustment
The reed/plate/tone chamber system is what we call a “formant”
The tone chamber acts on the reed just as the reed acts on the tone chamber.

We must therefore consider the resonant frequency of this space in the design of an accordion.

The material used to make the reedblock has no influence on the sound
Indeed, the reedblock does not amplify nor does it diffuse sound from the reeds even if it is able to transmit some energy to the soundboard in a small proportion, which itself transmits very little energy to the accordion’s body.

Easy to work with, we choose wood for its workable quality.
This video is a demonstration that proves the reedblock does not transmit or amplify any sound from the reed.
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