The 13th Amendment prohibited slavery and involuntary servitude, but did not prevent voluntary servitude. In other words it was lawful for banks and financial institution to turn unsuspecting individuals to voluntarily enter into contracts that would turn them into perpetual debt slaves,
When an individual borrows money, the collateral for the loan is the labor of the borrower. When the institution of slavery was legal in the 19th Century, the slaves food, shelter and the necessities of life were provided by their owners. Today the slaves of the 21st Century purchase the necessities of life with Federal Reserve Notes loaned to them by the banks affiliated with the Federal Reserve system.
There is very little difference between voluntary and involuntary slaves.
All of the people on earth are either masters or slaves. People give their consent to be slaves by obeying their masters. The people are governed by the fear of what will happen to them if they refuse to do what they are told.