But since you chimed in, why does it happen?
The short answer: Original sin.
Yes, there are a lot of "cultural Catholics" who engage in whatever sins but still abide by certain Church teachings. I'm glad for the fact that they at least will stick to something the Church teaches, anyhow.
We're all sinners, after all.
I think there is a pragmatic aspect to this, too, though. Catholics, even those who rarely if ever assist at Mass, wish to marry in the Church. And, because of the Church teaching on marriage, they can't do that after having been divorced, without an annulment, which can be a hassle to get (although they usually seem to be granted).