One definition of slavery is, working a day for a day's food. It's a condition almost impossible to get out of, as that involves starving.
"Similarities between wage labour and slavery were noted as early as Cicero in Ancient Rome, such as in De Officiis."
If you need the wage from your labour to feed and house yourself, such that you are completely dependent on your employer, how is that functionally different to slavery?
Unpaid internships are increasingly considered immoral for reasons that you identify - many people literally cannot afford to take such an internship, so cannot gain this experience.
Guess why