What reason do we have to think that for e.g. Google, marginal cost of hiring another engineer is not equal to the marginal benefit? Be careful not to confuse m

What reason do we have to think that for e.g. Google, marginal cost of hiring another engineer is not equal to the marginal benefit? Be careful not to confuse marginal benefit with average benefit, which as for almost all inputs in the economy, is always higher than marginal benefit.