Because there are much more unknown parameters in the outcome of AI for humanity than in your match against Stockfish.

For example, the timeline upon which AIs get effectively smarter than us is uncertain. Let's say you believe the probability this occurs before we solve the alignment problem is 80%, it doesn't seem too far fetched to think that in this case there is at least a 12.5% chance that AIs coordinate against us in a catastrophic way. Combining these probabilities you get a 10% chance of a catastrophic outcome for humanity.

Note that the numbers are not to be taken at face value, I just wanted to give an example of thought process which could give such a figure.