Possible reasons:
1. They get “beat down” (not physically) and want the immediate situation to be over with.
2. They get convinced they will be convicted no matter what and that a confession will bring a lessor punishment. (Unfortunately there may be some truth there. A judge may impose a lessor sentence if the defendant shows remorse for what he did which includes admitting the wrong. So if you are innocent and maintain your innocence but get convicted anyway, you can get more severe punishment than someone who actually commits a crime.)
3. They may have been drunk or otherwise impaired at the time of the offenses and get convinced that they did something they don't remember due to the impairment.
4. A long “interview” leaves them mentally, spiritually and physically debilitated and they make a mistake.
5. In colloquial, nonprofessional terms: Some people are nuts.
About 30 years ago I received a call from a detective asking if I would come down to the PD because he wanted to interview me about some crime that occurred in the neighborhood. I do remember him being surprised when I told him no, if he wanted to talk to me, he knew where I lived. Never heard from him again so I have no idea what it was all about.