I have some remote colleagues in another country and we discussed salaries recently. They're making significantly less than me, even though we do the same work. Is this fair because they live in cheaper countries?

Companies usually adjust salaries in different regions due to cost of loving/ purchase power. Other things may also come into play e.g. regulation. I moved from one country to another (same company, same position) and my gross salary almost doubled but because of cost of living, my cash flow after expenses was similar.

Fair? Not really. But common? Yes.

You’re confusing economics with fairness…..

Technically, because 100k here is like maybe 3-5x as much there.

Cannot say if it's fair or not. Imo, as long as it's good compared to the cost of living in that country, it's fine. Someone making 100k €/yr in Italy is considered fkin rich and, for example, could afford buying a new cool apartment every 5/10 years, just saying. Someone making 120k $/yr in NYC is just middle class and probably struggles a bit to get an apartment there

Companies create salary benchmark based on Cost of Labor, not Cost of Living. These are two entirely different things( though I realize why they shouldn't be). That is why you can't just assume what a salary is going to be by doing a currency conversion. It's more than that. Peers benchmarking, competitive market, etc. I am not saying it's right, but that is the rationale.