There are some who have accepted Christ who do stay in the Catholic church. Some do it out of desire to share with their fellow Catholics who are friends or relatives.
Some, especially older people, do it out of just not wanting to change at that point in their lives. Catholicism is all they’ve ever known and they are comfortable there.
I don’t doubt that there are genuine believers within Catholicism, just as there are genuine believers in ANY denomination. Likewise, there are unbelievers in any denomination. The only thing that would change between denominations is the percentages, the ratio between believer and unbeliever (those just playing church).
And then there are those who use the term 'born again' as some club they use to beat others down while totally ignorant of the point behind being born again. That point is mentioned at least 50 times throughout Jesus' ministry in the book of Matthew alone, yet their entire focus of their own state of Christianity rests in a term that Jesus used only once. But should I take it upon myself to declare them not to be Christian? I think not. I'll leave that one up to God.
I'll even go so far as putting some MORMONs in this group, too!