You're neglecting the fact that not every "poaching" attempt is successful and the fact that companies like Google have an incentive to keep salaries high in order to prevent poaching.
I didn't neglect that. How do these facts rule out that successful poaching imposes an additional cost per employee on companies? Paying employees a larger salary doesn't prevent poaching if your opponent can do the same. People are more likely to fail upwards and projects are more likely to cost more and be finished later because of successful poaching. Who bears this cost and how?