Why are only final goods and services included in the calculation of GDP?

Only final goods and services are included in the calculation of GDP to avoid counting things multiple times.

Explanation:

In the calculation of the GDP only final goods and services are included because the prices of the these already have the costs of all the things that were used to produce them. If we count the intermediate products and services that were used for the final output, we will be double counting which will give a result that is not a correct estimation.