Company A pays an employee $100 per day, and provides a lunch which costs the company $10.
Company B pays the employee $110 per day, but the employee has to provide her own lunch.
Is it really fair that Company B has to pay more tax than Company A? Compensation is compensation, whether in kind or in cash.
The problem with taxing a product is that the actual value is unknown. In your example, taxing the lunch as $10 of income is actually giving the benefit to Company B due to the "reverse inequality of exchange". IE if you buy a $10 lunch, you actually value it more than the $10.
For accounting purposes though, it's $10.
That flexibility might be more valuable to the employee than the nice lunch.