Most ex Catholics, I think about 7 out of 10 are secular or no religion. There was a study done on this a few years ago.
A good many other Catholics leave and join other churches if a spouse divorces them and they wish to remarry. The Catholic Church does not recognize divorce under ANY circumstances. They have a process called annulments where they construct a legal fiction that your marriage never really existed due to one of a handful of esoteric reasons. You cannot remarry and continue to receive Communion unless your marriage is annulled.
It can be a humiliating process, and many people just refuse to go along with the lie that their marriage never existed. So they leave. Generally when they remarry and get to their new churches they TITHE, which is driving the Vatican nuts. It has sparked a very intense debate about modifying the Church’s stand on this issue.
“Most ex Catholics” think they have a choice. Whoever is baptized as Catholic will be judged as Catholic. It’s not like divorce and annulment based on verifiable inabilities.
Not going to mass on Sunday for a Catholic is actually a mortal sin. Always was
https://www.ewtn.com/expert/answers/sunday_mass.htm