The wages are too low. Without assistance, they'd probably commit suicide, so you're logic is based on a false binary. A job that doesn't earn a base level of dignity is not necessarily worth living for, so people give up. It's very simple.
You can go "the wages are too low" all you want, but coming from a dead-end shit town, with minimal life prospects and dropping out of high school to having a GED and training with certifications in a field that pays above the median single-earner income nationally is a pretty good deal. I think Job Corps is a good program, I also think a lot of people who go into Job Corps are doing it as a last-ditch effort but don't actually have any desire to take responsibility for themselves or actually work.
Every single job that Job Corps trains for pays a fair wage in most markets. While it'll likely never make you rich, it is certainly well into the "base level of dignity" territory. Clearly the people I know aren't tempted to commit suicide, they're still investing as much time as they can when they're not wasting their life away playing World of Warcraft to scam government benefits, so.
You aren't treated with dignity by the customers, by your boss, or by the size of your paychecks.
I will say that it's a damn shame that it's not available for someone like a 40-year old who wants to get their life on track.