Why is it necessary for people in the Republic of China (Taiwan) and Japan to import wood products?

Because they do not have the competitive advantage to produce them by their own.

Explanation:

Taiwan and Japan both have their own forests that can be used for lumber products.

But, exploiting the number of these forests do not even close to other major wood producers in the world. Brazil and Russia for example, have enough resources to produce the wood at significantly lower price compared to Taiwan and Japan.

Because of this, both Japan and Taiwan focused their resources on manufacture, service, and finance industry on which they are significantly better compared to most countries in the world. To fulfill the demand for wood products, it is cheaper for them to just import them from other countries who have the competitive advantage.