A forecast is an estimate of the probability that a prediction will be true. It is a number between 0 and 100, where 0 means you think there is no chance whatsoever the prediction will be true and 100 means you think that it absolutely it will be true.
A forecast is not a prediction. It's not an either/or proposition. It is the estimate of the likelihood that a prediction will turn out to be true. Another way of putting it: how much of a chance, percentage-wise, would you give this prediction?
In short, because few things in life are certain. There are always unexpected events that could occur that affect the outcome of a projected event.
Also keep in mind: you are awarded ForeKarma based on your forecast. This site will track your progress, so making more precise forecasts that take into account doubts and contrary views will improve your reputation over time.
For more information on karma, see the Karma FAQs page.
A score of 50 is safe, but it says basically, "I have insight whatsoever to offer on this prediction."
The point of this site is to encourage you to think more critically about predictions: your own, those of others, and especially those of public pundits.
You are also encouraged to do research and learn more about the subjects you are forecasting. This will make you more knowledgable generally, a better critical thinker, and improve your forecasting ability.