How do people get into massive credit card debt?

it's 20% added every year. $40 become $48 after one year. After two years it is $57.60. After three years, $69.12. at four years, $82.94. Of course a person is paying off part of the bill each month, but they are also adding on more expenses each month. Also, some people's rates are well above 20% a year.