Short explanation: Some might not.
More complete explanation:
An actual, conscious, fully-willed rejection of Christ and His Church is called “apostasy”. A true apostate will most certainly wind up in hell (unless he repents before dying and returns to the Church and to the state of grace). But not everybody who “leaves the Church” is an apostate. Here’s why.
Most people who “leave the Church” have such a muddled understanding of the Church that their decision does not involve an actual rejection of Christ or His Church. They usually think of the Catholic Church as just one option among many, and they blithely go “church shopping” (sometimes called “church hopping”) looking for a religion that makes them feel good, as if religion were merely some sort of self-help program or emotional support group. Since these people have such a grossly inaccurate idea of what religion is all about, they are not guilty of apostasy.
Likewise, many Catholics disagree with one or more of the Church’s teachings and “quit” because of that. For example, the Church’s clear stance against artificial means of birth control has caused many Catholics to seek less demanding religions. But these people are not (usually) guilty of apostasy either, since their leaving is based entirely upon a misunderstanding. The sin of apostasy requires a ful understanding in the truth of the faith followed by a rejection of it. That’s not what we have in this situation.
Also, think about all the people who “switch religion” due to marriage. A long-time friend of mine has a brother who attended BYU and fell in love with a Mormon lady. Since the Catholic religion allows mixed marriages, but Mormonism does not, he “converted” to Mormonism and married her. This sort of thing happens all the time, but is rarely true apostasy, since again it reveals a gross misunderstanding of what religion is all about.
Parents often ask me, with tears in their eyes, what they should do about their adult children who have left the Church. I always recommend that they continue to teach by quiet example, to pray for their children, and (here’s the tough part) to regularly extend a polite and tension-free invitation to go to Mass together. Parents should never act as if it doesn’t matter what their children do (it does matter), nor should they go to the opposite extreme and threaten them will hellfire and damnation (see above for why that’s not appropriate either). And pray to Saint Monica, whose years of prayers and tears for her wayward son finally “paid off” when he converted and became the great Saint Augustine.