Because even deportation has legal hoops you have to jump through. If they get deported, even willingly, what happens when they come back to American on another

Because even deportation has legal hoops you have to jump through. If they get deported, even willingly, what happens when they come back to American on another vessel? Their special visas to disembark while at port would most likely not be approved if they have a big "deported" note on it. On top of that, a lot of these workers are kept in exploitative working conditions and are owed thousands of dollars in backpay. Leaving the vessel to be willingly deported effectively negates any chance of actually being paid and can kill your career right then and there. I highly doubt that's a risk any of them are willing to take.