Well to answer that question - we first have to understand the difference between the two terms.

Wages - Wages are mostly associated with production employees. That means the workers and employees who are directly involved in the production and manufacture of products.

Salaries - Salaries are given to workers such as professionals, managers,researchers, etc., who work to run the day to day operation of a company but are not involved in the direct production of goods.

The rule of accounting states that since wages are directly affecting the cost of goods sold, that is why it is taken in the calculation of COGS and not taken as an operating expense. Salaries however do not directly affect the COGS and hence are accounted for under the heading 'Operating Expenses'

In addition to Manisha's answer, wages vary with production hence direct impact on revenue(remember your 'matching concept' in accounting) while salaries are fixed, that means you must pay whether you generate revenue or not that is why salaries are classified under operating expenses.