It isn't, which is why the 13th amendment (which bans other forms of slavery) explicitly permits it.

Some members from both houses of Congress have recently proposed a constitutional amendment to change this:

https://www.merkley.senate.gov/imo/media/doc/The%20Abolition...

Because it isn't. It's distinct from chattel slavery, which is not the only form of slavery. Bonded labor is the most common form of slavery today, and is also not chattel slavery.

Except for the due process trial, conviction, and sentencing.