We keep losing costume designers. The good ones go off and end up with... real jobs. We can only grab them and suck them dry briefly, then they find that they have skills that people will pay for, whether that's Broadway musicals, directing shows out-of-town, or much bigger movies.
Same thing happens to us with boom operators. Or at least to me with boom operators. You train someone to do boom and then... well... BOOM. They have a marketable skill and can work. Which means we're off the boom operator gravy train.
But that's not what's important. On Reddit they have an "Ask a 4-year-old anything" thread.