haypoint is correct about the "migrant" or "undocumented" worker (illegal immigrants or resident migrants) issue. When you have people who will work all day for low pay and no benefits why would you pay minimum wage and benefits to people who won't work as hard? But there is another aspect to the story. The attitude of some, probably most, young Americans is "gimme, gimme, gimme". Most American youths today have no idea how to work, show up to work on time or even do the job they hired on to do.
I had the experience of trying to train people how to do a menial, low skill job which paid better than minimum wage. I could tell you within 2 hours if a person was going to be there and collect a check, or actually do the job. Most Americans, especially young women, thought the job was beneath them and they shouldn't have to do it the way I showed them to do it. It was just fanning through mail to make sure an address was in the envelope window and making sure the envelope was sealed then putting the mail in the proper tray. Wasn't hard, boring at times, but not rocket science. I gave my trainees all the help they asked for. I wasn't mean about it but I expected them to do the work and stay off the phone. Very few returned the next day, even fewer would come back the next month.
But maybe I'm just too hard to get along with, IDK. Several of my machine operator trainees stayed for years.