Project Innocence helps exonerate innocent people. They claim that about 30% of their “clients” had admitted guilt. My understanding is that it’s mostly based on DNA evidence so not really a matter of opinion: https://innocenceproject.org/exonerations-data/

“Why” seems to be essentially coercion from the interviewers. People told that if they admit guilt then they’ll get a slap on the wrist and be let go, else go to costly trial etc.