This is a very simple question if we consider possibility theory with the (new) definition of an event without a cause: an event with exactly zero factors limiting or favoring its appearance.
There are no factors influencing the possibility or necessity of the event, so they remain at zero, and thus the probability of the event remains at zero as well.
This is coherent with what we seem to observe. Random things could happen constantly but they don't seem to happen. Any event with a low probability should happen sooner or later given enough time. Why don't these things happen? Because the probability is zero, at least in the macro-world.
In the quantum-world things seem to be different, but the counterintuitiveness and the paradoxical nature of quantum physics leads to think that either ontologically the world is in a way we cannot understand or epistemologically we have not been able to understand how it really is. Which in short means that either the paradoxes are real or in our minds (due to a misleading framing).
The only event that is special in this sense is the creation of the Universe. Probability works in a different way here. We cannot speak about the half-life of nothingness to become a Universe because there is no time without a Universe. We cannot really ask how long did it take for the Universe to start existing.