If you’re in high tech (FAANG) the salaries have been going up so quickly year by year since 2000 that staying in one company for more than a few years is often irrational if you’re trying to maximize salary. Companies will gladly pay you an extra 50-100k a year just to steal you from a FAANG.

Your points make total sense though. Companies lose out by not having employees who have built up institutional knowledge, but in today’s market most of the people who companies want the most are likely to leave for greener pastures in 2 years.