Why missionary efforts are doomed

I’m tired of being told i have to be careful about teaching my children correct gospel principles, because otherwise they’re guaranteed to be lost (in the religious sense) forever. I mean, it just doesn'’t work logically—consider:

1. Assumption: If children aren’t taught correct gospel principles, they won’t live according to them when they grow up.
2. The only reason that assumption could work would be if there’s no way someone who’s not taught gospel principles in childhood would follow those principles when they’re older.
3. Therefore, the church’s missionary efforts are doomed because conversions to the church are completely impossible, since the missionaries’ efforts largely target those who weren’t taught gospel principles as children.

Or, in other words, reductio ad absurdum.

More seriously, i still don’t get why we have so much trouble coming to terms with the idea that people make their own decisions about their spiritual lives, even those who were raised in the church.