It's just an expression. Or perhaps they want to validate their own personal choices. It would surprise me to learn that they indeed worship fate - whatever that is.
I agree with Katherine.
"It wasn't meant to be." can simply mean that there are things that are out of our control. And if it is God's will that someone not marry, for whatever reason(s), and they choose to follow the Holy Spirit's leading on that, they can still accurately say that it wasn't meant to be without being fatalistic.