It isn't, but I'm pretty sure the status quo is a time consuming interview with lots of puzzle solving coupled with a probationary period and a vesting cliff. I think because hiring is so difficult (on both sides of the table) that people get cold feet with marginal performers during the probationary period. Employees are reluctant to leave - interviewing is very hard, and managers are reluctant to fire - sourcing is very hard.

My thesis is that if interviewing was easier, both leaving and sourcing would be easier and everyone would win