Consider even a relatively trivial interaction: frustrated with the village blacksmith, my PC berates them. This could result in the blacksmith relenting and giving my PC what they want. Or it could result in the blacksmith bristling, and freezing my PC out. Either is possible. If the GM is at liberty to choose which (or whether some other reaction I haven't thought of in this post) occurs, how is the player to know which will occur?
If you,
@pemerton, berate a local mechanic, this could result in the mechanic relenting and giving you what you want. Or, it could result in the mechanic bristling, and refusing to do further business with you. Either is possible. If the mechanic is at liberty to react in either fashion (or in some other way you haven't thought of), how are you to know which will occur?
This is not in any way intended to be a smart-alec response. I genuinely don't understand why you expect that, in this hypothetical sandbox where the interaction with the blacksmith is taking place, you should always know what the exact outcome of your actions are before the outcomes occur, any more than in my real-world restatement of the scenario. Just like in the real world, consequences follow actions, they don't precede them.
If you know the blacksmith well, or the character makes an effort to gather information about the blacksmith's demeanour, or picks up on some kind of cues during the conversation, you may be able to predict the reaction. But if you just launch into a berating, you won't know how they will react. You (as a player) have chosen to throw caution to the wind and accept consequences you can't predict.
As GM, I will take whatever information I know about the blacksmith, consider the the way you go about berating thenm, contemplate possible responses and, quite probably implement some kind of mechanic to guide my decisions (whether an old school style reaction roll, an intimidate skill check or what-have-you) and combine those to make a call and then roleplay the blacksmith's response.
If you're a tough guy and you know that blacksmith is easily intimidated and you've got something specific and embarassing to throw at them, they'll probably back down. In such a situation, the player can be confident of such a response -- they know the blacksmith is likely to back down for the same reason their character knows it (although there is always a very small chance the blacksmith chooses this day to grow a spine; if you go around antagonising people, you have to consider that at some point someone might bite back).
But, again, if you go in blind and berate someone you know nothing about, you probably won't be able to predict the response, and that is the game functioning as intended.