If you think one of them is immoral and unfair, but the other isn't, you're the one who needs to square that cognitive dissonance - not me.
It sounds like you generally think economics should trump the welfare of existing residents. That's certainly a view, but a logical consequence of it is wide-open borders.
No, you are. You have imaginary rules about who should be protected in a city (based on seniority??) that nobody else is aware of and are certainly not following. It goes by things like age and net worth, and basically nobody gives a shit how long you've lived somewhere.
Rent control is not imaginary (Any more than any other rules are), and yes, that is exactly how it functions, for the purpose that I have described.
It advantages senior occupants (Who are grandfathered into controlled rates), at the expense of junior ones, or ones who don't live there yet (Who are presented with inflated rates, for obvious economic reasons).
Why should you deny him that mobility? Is he any worse a person than you are? Why does he have to jump through hoops and quotas and queues and all sorts of degrading, dehumanizing bullshit before he can do so?
Why?
---
If your answer is 'because that's the law and because we can make him jump through all those hoops', that's the answer to why cities choose to protect people who are existing residents at the expense of people who aren't. Because it's the law and because they can, if you don't like it, tough, you are free to go somewhere else.