Why are there so many mentally ill inmates in prisons and jails across the United States?

Around two million people with mental illness are booked into jails each year. Nearly 15% of men and 30% of women booked into jails have a serious mental health condition.
Why are there so many mentally ill inmates in prisons and jails across the United States?
Textbook:
Corrections in the 21st century (8th ed.) by Frank Schmalleger & John Ortiz Smykla, McGraw-Hill

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