Ah, I get it now. Because the DM is narrating the outcomes of the check, not the behaviors of the PCs.
I mean, let's take the Frightened condition as an example. Even if the DM said that the result of a check imposed the Frightened condition upon the PC, then what? The PC would be bound by the mechanics of the Frightened condition: disadvantage on checks and attacks while the source of fright is within sight, and can't move closer to the source of fright. None of that infringes upon the player determining how their character thinks, speaks, or acts. They might think or say "I ain't afraid of no source of fright", but so what? They can't do certain things as a result. They might try to act brave and move closer but, again, so what? The just because you attempt to do something doesn't mean you can't do it. The player is still deciding on their PC's behaviors - thoughts, actions, speech. The outcomes of those behaviors are constrained now however.