In my experience, the reason for this is deep specialization. Even for relatively simple jobs, it's very time consuming to get someone up to speed. For complex

In my experience, the reason for this is deep specialization. Even for relatively simple jobs, it's very time consuming to get someone up to speed. For complex jobs it's sometimes downright impossible. Bosses respond by heaping more work on the employees who already know what to do and can achieve it efficiently. You should respond by asking for a raise. Be tactful, do it a few times and then use it as an argument. It really doesn't matter whether they pay you for overtime explicitly or as part of your base.