Staff who’ve been around a while, understand how a company operates and can seed that understanding into new staff are more valuable to companies. For example: if every worker were replaced with an equally skilled worker tomorrow a company regardless would not be able to function. It therefore makes sense that a senior employee can demand a higher wage [than a new starter] even if their direct productivity is no different and so a gradient in wage for seniority is exactly what one would expect to see in a free market.