I think job hopping for salary increases is a byproduct of companies being unwilling to pay employees more.

Companies don't want to reward long time employees with money. They are more interested/beholden to their shareholders.

If companies regularly provided more than just cost of living raises, improved working conditions, etc they would have more worker retention. But as it stands, the best way an employer can tell a company that they are undervalued is to (threaten) to leave. Otherwise you keep improving, but your paycheck stays the same. You can ask for a promotion, but unless they think they'll lose you, they have no monetary incentive to give you that raise. And that's when you get employees strung along. As one redditor put it, a promotion delayed is a promotion denied.

TL;DR: Companies don't want to pay employees more, so employees find other companies that will.