Why is timber expensive?

First of all, trees grow on land which if it were not used to grow trees, would have other economically valuable uses, such as farms, housing developments, industrial parks, etc. Secondly, it takes years, even decades (depending on species) for a tree to reach full maturity, when it can most profitably be harvested for timber. Then the tree has to be cut down and taken to a lumber mill to cut it up into lumber, which also costs money. So there is an investment of time and money that goes into making timber. Beyond that, prices reflect the interaction of supply and demand, in a free market economy. There is enough demand for timber to keep the prices in a certain range. If people needed less timber, it might become a glut on the market.