They do realize it about themselves, that's why they assume everybody else is. Because they're so busy doing it and it's so natural and ordinary for them, then therefore everybody else must be worser because they're bad people who aren't saved by Jesus.
It's like our favorite Karen and the rest of the Bible Belters assuming that us Northerners must be exceedingly foul in our sexual habits, because if they do what they do and they're the ones with God-given morals, then of course everybody else must be so much worse than them in what they do.