What About The Prodigal Son?

It still baffles me that so many Christians subscribe now to the "once saved, always saved" doctrine, where if you don't believe anymore, you were never saved in the first place.
When I was a Christian, growing up in the late 90s and early 2000s, I heard about people who had "wandered away from God" and "rededicated their lives to Christ." Where has this rhetoric gone?  As far as I can tell, it has been an important part of the bible and Christian doctrine for centuries, millennia even, only to largely disappear in the last ten years.
I credit the rise of atheism and skepticism with a bit of the shift, and Christians trying to comfort themselves that they will never "fall away" like us, but that does not seem important enough to explain it all. Why aren't all of these newly-minted atheists merely "wandering away" from God, and not officially "falling away?" Why are they a devout and true Christian one day, and then suddenly they were never sincere to begin with? Doubt seems to be the only sin that can never, ever be forgiven.

This doctrine also goes against the bible. Check out Ezekiel Chapter 33: 

…12"And you, son of man, say to your fellow citizens, 'The righteousness of a righteous man will not deliver him in the day of his transgression, and as for the wickedness of the wicked, he will not stumble because of it in the day when he turns from his wickedness; whereas a righteous man will not be able to live by his righteousness on the day when he commits sin.' 13"When I say to the righteous he will surely live, and he so trusts in his righteousness that he commits iniquity, none of his righteous deeds will be remembered; but in that same iniquity of his which he has committed he will die. 14"But when I say to the wicked, 'You will surely die,' and he turns from his sin and practices justice and righteousness,…"

And nearly everyone is familiar with Jesus' stories of the lost sheep, lost coin, and the prodigal son. 
Why are so many Christians ignoring the message of the prodigal son? It's as if the older brother, when his brother left their father for distant lands, said, "Well, that just goes to show that he was never really your son to begin with."
Everyone knows that that would be absurd, and yet it doesn't seem absurd to them to say, "You were never really a Christian to begin with."
I do think that their attitude is rather vindictive, though. Sometimes it did seem easier just to stop trying, just to "leave God," when I was a Christian. I could see Christians resenting those that they see as having taken the easy way out. In a strange way, it's like Martha in the New Testament, who was mad at her sister Mary for spending time with Jesus rather than helping her with endless preparations for a feast. If their relationship with God is work to them, then they would resent those who don't work at it, who don't do their duty, as it were.
I think a lot of why they dismiss former Christians is resentment, combined with fear that they too will "fall away from God." That doesn't mean, though, that we have to be nice to them if they're not nice to us, or that they don't need to be reminded of what their own bible is saying.