[dylan604]

Some companies have internal policies that set hard limits on the increase an existing employee can receive per year. Those obviously do not apply to a new hire, so the salary negotiations are expected at that time. This has been the reasoning for the "if you want a raise, get a new job" concept that I have always understood.

[Fanmade]

That's indeed part of it. Another part is that working at another company usually gives you new perspectives and you learn more. So having been in a lot if companies and in turn having seen a lot of different approaches counts towards your experience, which also justifies a higher salary.

I actually got a 10k raise in one company, but only because I really didn't earn enough before. When I quit a year later anyway for another job (which offered me another 14k raise), my employer made a counter-offer of 30k. I did not take it, but at least it gave me a perspective how much I apparently was really worth to them.