Individual perception of fairness being mutually exclusive in aggregate probably have to do with it. To the old being paid the same as the young can be viewed as unfair because they do a better job.
Younger workers can see older programmers getting paid more but don't neccesarily see much "better" from them (who is right varies by individual and field of course). It can come across as gatekeeping in "paying their dues" instead of actual capabilities or lack of them holding them back which breeds resentment.
Of course at large there are configurations every which way in practice across varied workforces: underpaying older hires in bias of new, overpaying old in bias, putting arbitrary caps on promption to upper management regardless of prior successes for being too young, refusal to promote elders for fear of their retitement disrupting despite the project being short term and all sorts of dysfunctions at scales small to titanic.
Younger workers can see older programmers getting paid more but don't neccesarily see much "better" from them (who is right varies by individual and field of course). It can come across as gatekeeping in "paying their dues" instead of actual capabilities or lack of them holding them back which breeds resentment.
Of course at large there are configurations every which way in practice across varied workforces: underpaying older hires in bias of new, overpaying old in bias, putting arbitrary caps on promption to upper management regardless of prior successes for being too young, refusal to promote elders for fear of their retitement disrupting despite the project being short term and all sorts of dysfunctions at scales small to titanic.