Why does the industry optimize for finding a new job every ~18 months to stay competitively paid rather than promoting growth within a company? Is there an efficiency I'm missing? Constant attrition, recruiting, interviewing, negotiating, onboarding, etc seems inefficient at best

TLDR feeling undercomped makes employees feel bad and they'll leave and companies will then have to pay market rate for their replacements but most companies won't do market rate adjustments because it's hard to justify spending money and quantify costs of attrition

That's the most well assuming answer at least. There are likely a lot of bad reasons and justifications. How old were you when you realized you were just another corporate resource to be exploited?

@ the people replying that you hire people and job hopping is bad and only for greedy people with bloated egos and your employees never leave because they're under compensated - try listening it's really great.