It would help if the question was a bit more specific.
What cohort level are you speaking about? (i.e. partner, manager, consultant, etc)
What practice area / functional specialization?
At more senior levels, the exit options become more interesting. At junior levels, not so much.
On the whole though, you're right. There is a revolving door of floaters. The talent grade is mostly similar so it makes sense to get into the poaching game to beef up critical mass and increase billings.