In the sentence
He should get a wooden table, not a plastic one
why cannot "wooden" be replaced by the word "wood"?
In the sentence
He should get a wooden table, not a plastic one
why cannot "wooden" be replaced by the word "wood"?
There is no reason why it can't in this context. Both "wood" and "wooden" can mean "made from wood".
Use the wooden/wood spoon to stir the soup, as that won't damage the pot.
However these words are not interchangeable. "Wooden" refers specifically to those things which are manufactured from wood (or which metaphorically appear to have been made from wood).
It's an old wooden doll that has been in my family for centuries.
"Wood" can mean the same, or it can refer to the base or source of the thing:
How to stoke a wood fire stove
Our real wood paper is made from actual very thin wood veneer and a paper backing.
Unfortunately, a lot of this seems to be idiomatic. For example, both "wood board" and "wooden board" are natural ways to describe the same thing, but "wooden paper" sounds different from "wood paper".