> Given how easy high-skill employees jump ship today, I can see why companies would naturally become this defensive.
High-skill employees jump ship because the old school system of promotions etc. is all but dead. The days when people used to work for one company for 30 years, gradually moving up the ladder, are over. The vast, VAST majority of increases of pay today are due to people moving from one employer to another employer. The widespread corporate culture of refusing to significantly increase pay of existing employees is to blame. If the only option for people to get these double digit % pay bumps is to leave, then they will leave.