We put people in prison for longer periods of time and for less serious offenses than any other industrialized nation. Per capita there are more Americans in prison than any other nation in the world which is a severe problem in American society. Longer prison sentences don't deter crime, they just cost more and hurt society.
Percentage-wise, too many Americans are in prisons, because the United States has a high crime rate. The violent crime rate (murder) is unnaturally high compared to other developed countries. Prisons are used in the United States for two reasons. One, is to keep dangerous people off the streets and from committing more crimes. Two, is to send a message to deter others from committing crimes. Because of the high crime rate in the United States, sadly, there have to be a lot of people in prisons.