Sure, that's a good question, and a valid point. The PbtA games I'm familiar with may have additional mechanics to add to this (for example, Stonetop has Advantage/Disadvantage very similar to 5e). The GM also has the ability to say something is not possible, or that it may take more than one success to accomplish (again, this may vary a bit by game).
So, the example you gave of convincing a king to give you his kingdom isn't going to work, unless there's a compelling reason that it may. Same for simpler tasks; you generally aren't going to roll unless there is something at risk.