Employers don't reward long-term employees because they don't need to. Even though everyone knows you make more by job-hopping, companies are following a rational strategy because too few people "walk the walk" despite wanting more salary. Arguably unethical, but rational.
At the cost of losing a minority of job-hoppers they retain the cheap majority that:
- finds job interviews exhausting, or is anxious about being rejected
- dislikes conflict or negotiation
- has good relationships with colleagues they don't want to lose
- has a comfortable commute or WFH arrangement they don't want to change
- is proud of becoming an expert / go-to-person in some part of the business and doesn't want to lose that source of social capital
- is proud of product or service they have made a big contribution to and wants to "see it through to the end"
- believes leaving would place an unfair burden on other team-members or would be disloyal to a manager they consider a friend
- (US only) cannot risk losing healthcare
It's possible that in the future the ratio of job-hoppers to lifers changes and companies find they need to switch strategies. But until that happens the job-hoppers will have the upperhand compared to the lifers.
It's the same reason why your phone/insurance/whatever provider puts up the prices year-after-year: although some people leave, enough people simply put up with it that they make more money this way. "Do nothing" is an easy choice and companies are banking on enough people making that choice.