In a game, if another player has an equal modifier to you, he will tie or beat you 52.5% of the time.
If his modifier is 1 lower, he will tie or beat you 47.5% of the time
if it is 2 lower, 42.75% of the time.
and 3 lower, 38.75% of the time.
This creates a feeling of futility: At 3 lower, from levels 1-4, that means you could be trained in a skill, AND have a higher modifier to the stat, and still roll lower than your teammate more than 1/3 of the time.
Your 20 Charisma trained in Persuasion Bard is going to lose to the 18 Charisma sorcerer a real a palpable amount.
Now watch that happen with every initiative roll, every attack roll, every save, every skill check. The things you trained it will fail you while the lucky idiots succeed, again and again and again.
It creates a feeling of futility. Of uselessness. Why should I do anything except the one thing I am AMAZINGLY better at, if almost half the time, I lose to the guy who rolled better than me at chargen?
And I've watched it play out in the games. I've seen the ranger announce their 18 Perception proudly, and then the druid says "21".
And then you watch BOTH of them fail a DC 15 check, and the Goblin fighter rolls the 18 to beat them.
No one feels like they're good at anything, because statistics means someone else is beating them AT THEIR TRICK close enough to feel like every other time. And you watch the players instinctively react: they try and make new characters to bring to the next adventure, because "This one just doesn't feel like he's adding much," without noticing that half the group is saying the exact same thing: NO ONE feels like they're adding much.
Granted, the group I watched it all happen to had too many players (6-7), which accelerated that sense of uselessness descending, since there were just more rolls happening, but that's the real experience: your character is trained in a thing, you have backstory invested in your ability to do X...and it means nothing.