Why closing stock is not recorded in trial balance? What's the concept behind it?

Closing stock is the leftover balance out of goods which were purchased during an accounting period but are unsold till now. Total purchases are already included in the trial balance, Hence closing stock should not be included in the trial balance again. If it is included, the effect will be doubled.

because stock concept is related with used material. and when ever we going to project we always required new material. that's all.