If the reliability stays constant expressed as a rate (arguably optimistic), like ".1% failure per X flights", and the # of flights increases, then failures become more frequent.
Failure rates must decrease proportionally to the utilization/capacity increase for the # of failures to stay constant. Otherwise, the # of failures will naturally increase, it's simple multiplication.
For example:
If crime rates stay constant, but your population increases, you have more crime - not the same amount of crime as before. you have more terrorists, you have more school shootings, because your population doubled in size and you didn't cut the rates of abhorrent behaviors in half.
Air travel has _exploded_ in popularity, this is why you see all sorts of things more frequently than you did before. From reports of airline passengers being dragged off planes in totally asinine scenarios to deaths of pets in the cabin.