A person certainly can and does leave Catholicism.
I do not practice any Catholicism any more.
Jesus set me free from bondage to religiosity.
+1
I do not practice any Catholicism any more.
I think the point (and frankly I can't believe you really don't understand it) is that an "ex"Catholic merely has to return to the Sacraments without fanfare whereas someone who is converting has to go through the process.
When you hang around with priests you hear a lot of "X came back to the Sacraments after 40 years today" type comments. People do it all the time... a lot. Your Family is waiting for you to come home is the point.
+1