You drill GTO Wizard, and it tells you that a particular river bluff-raise is roughly 0 EV. Doesn't matter if you do it or not – same result either way. It’s breakeven.
So you don't do it. Why would you? It's a little scary, it's a little uncomfortable, and the solver just told you it doesn't matter.
You do this with lots of similar spots – big river bets, bet-3-bets, IP and OOP. Each time, solver-approved, zero cost.
But the solver isn't who you play against tomorrow – you'll (hopefully!) be playing a person. And that person, if they’re reasonably sharp, is going to start noticing something about you.
Your range is starting to get out of whack. You're not bluffing the river enough. And once they pick up on that, the adjustment is simple – they just fold more when you try to put a lot of chips into the pot.
I have seen this in students and opponents. Someone drills against the solver and looks great – losing very little EV. And then they sit across from someone who's paying attention (hopefully me) and they get exploited.
And they can't find the leak, because in the tool they're using to study, each individual mistake isn't a mistake in isolation. But this bunch of non-mistakes, piled on top of each other, magically turns into one huge leak.
The tool you're using to study isn’t yet designed to keep an eye on your range. Your opponent is, and you should be, too.