Why? The Penal labor exemption is the one case where slavery or involuntary servitude is still permitted in the US constitution.

For one, most people don't equate indentured servitude with slavery. We generally think of lifelong service when we say slavery, which isn't part of the penal system. The penal system also isn't generational and people aren't born into slavery. There's grounds to call it slavery, yes, but the context you're bringing it up in is in comparison to the African slave trade and you're diminishing the suffering those people went through by saying that what happened to them was just like what we do to prisoners today. What happened to them was much worse.

That's not really correct from any reasonable interpretation.

I may missing something but it seems plain enough in the thirteenth amendment? “ Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.”

To this point, if US prison labor isn't "slavery" or "involuntary servitude", then removing the words between "except" and "shall" would be noncontroversial.

Similarly, if there was political consensus that US companies shouldn't own or hire slaves overseas, then the bit starting with "within" would be easy to remove as well.

None of this has had any chance of happening since the thirteenth amendment was passed (not even during BLM). It's pretty clear that a big chunk of the US's leaders are pro-slavery.

Because worrs carry not only direct meanings, but subjective connotations, and most people consider slavery an unjust subjugation of another human being, and think it is immoral by definition, in any circumstances. On the other hand, even most of the people who aren't fans of US prison system still consider the general idea of prison to be just, as the general idea of prison labour as a way to repay society.

Also if you're in prison and refuse to work, what are they going to do, send you to prison prison?

Idk how this works in the US in particular - but I suppose that - essentially - yes. Harsher conditions.

When one has a essentially complete control over another person's life, there are ways to make this life hell, even while staying within the legal bounds.

The same stuff that happens when you don't comply in other ways in prison?

From my understanding, the main punishment for not working as prison labor is losing the small wages you do get (i.e, normally one of the punishments for misbehavior is prohibiting you from working).