Why is it okay for one side to release 40 hostages and expect the other side to release hundreds? This happens all the time: some American who did drugs gets t

Why is it okay for one side to release 40 hostages and expect the other side to release hundreds? This happens all the time: some American who did drugs gets taken by Russia or China, and to get that person back, we give them 10 hardened criminals or spies. I get it: if it were my relative, I'd want them to do the deal too. But it just doesn't seem reasonable at all.