Empirically, companies are very bad at evaluating this cost. If they weren't, upwardly-mobile job hoppers wouldn't exist because the previous company where they

Empirically, companies are very bad at evaluating this cost. If they weren't, upwardly-mobile job hoppers wouldn't exist because the previous company where they generate positive value would have increased their salary over the salary they could get elsewhere. In practice we see that the companies are reluctant to give raises, hiring costs are considered capex, and there are inefficiencies in salaries because frictions cause employees to stay in roles that are not financially optimal, meaning there's a premium to accepting the risk of moving jobs.