To deport someone, the receiving country has to agree to accept them. Mexico will accept their own nationals, and possibly those of some other countries, but they won’t accept everyone. What do you do with people who’s country refuses to accept them back (this has happened)? And what do you do with them when you can’t determine where to send them back to? You’ve already promised they won’t be let loose, so you’ll put them into containment facilities - all very hastily-built, because there are so many people you’ve accumulated so quickly.

But you have more people than you know what to do with, and they’re all just idling in the camps, and the nation is desperate for workers. The temptation is too great: instead of spending all this money maintaining the camps, you can offset the expenses by renting them to various industries as cheap labor.

It’s still expensive, your people still resent spending money on them, and you still want to deport them, so you don’t spend much on food or medicine or clothes or sanitation. Winter comes, disease rips through the camps, and masses die - but that’s not your responsibility, these things happen …

Just a reminder that most of the concentration camps Germany built weren’t extermination camps, they were slave labor camps but the administration didn’t care if the prisoners died of disease or injury, they’d just round up more people to feed the German machine.