Why is silicon used as PCBs?
PCB's are NOT made of silicon!
PCBs are made from coated paper (coated to make it somewhat fire resistant), this is the cheap option.
Better PCBs are made from epoxy coated fiberglass, a common material is FR4.
Reasons not to use silicon for PCBs:

Silicon is only a good isolator when it is very pure (no contaminants). For making semiconductors extremely pure Silicon manufactured. This is expensive to make but not much of it is needed per transistor / IC so the total material cost per device is quite low.

Silicon is very brittle it is simply unsuitable to make a strong PCB that can withstand some mechanical force.


Reasons not to use wood for PCBs:

Wood is flammable

Wood is not strong when it is thin.

Wood can bend over time due to aging and moisture.


Coated paper and FR4 do not have the disadvantages mentioned above. Also, it is easy to coat them with copper to make the connections. It is easy to drill holes in the material. These materials are cheap.