I believe he means there's a big commitment to get somebody going into a project. To create value, engineers need to be involved in many aspects of the product, which takes time for both parties, which is paid by the employer.
It's easy to see if someone is incompetent during their first 90 days: that's a basic recruiting failure. It's the other type who are sort of good, but ultimately disastrous, that cause real problems. And often, these mis-hires take ~12 months to identify and correct. The result is that caution dominates when hiring.