Say we considered a simplified case and focused on a single engineering team that is solely responsible for a single product. Just looking at the revenue generated by that product is insufficient since it excludes the contributions of sales, marketing, business development, etc. in driving that revenue. And practice most engineering teams are dependent on other engineering teams for providing shared services and infrastructure.
Its also insufficient to just consider current revenue of a product. We also need to consider the potential future gains from a product, which is unknown.
And quantifying value add gets even harder when we start looking at individual people. Considering individual contributor engineers, there are numerous ways that one can contribute, ranging from developing new features, diagnosing and fixing bugs, writing and updating documentation, mentoring, interviewing candidates, etc.