Which is odd in and by itself. Whenever you have a review with me as your employee and we discuss salary, consider this as a from-scratch negotiation. Just as if I just had quit, left the company, and now somebody new (me) showed up at the doorstep, about whom you have from a very trusted source super-reliable info about their capabilities, work ethics, etc. How much would you pay that person? If the answer results in "10K more than you today" then that's the raise I'm expecting. Anything lower is trying to trick/cheat me and banking on momentum/laziness on my end. Which ultimately would be quite offensive. So, just offer me that money and explain why. (Same goes for salary reduction, obviously. Better that than building a grudge and then suddenly firing me.)

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.

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.