I remember reading this years ago. Part of it is that the distance between the sensor and the bit of the fly’s nervous system that ‘makes the decision’ is much shorter so signals are quicker. Also, some reactions are hard wired and require no thinking time.
For example a bees wings continuously flap unless their legs are on a surface. They don’t need to think about flapping their wings just on jumping up to take off or steering to land.
In humans, it’s possible to measure the time it takes between the brain making the decision to swat and the signals reaching the muscles involved. So unless you are good at estimating where the fly will be then you’ll miss.
Reminds me of updating an old Nokia feature phone (an XpressMusic) using Nokia PC Suite.
The progress bar just kept going until it hit the edge of the window (probably clipped by Windows). The percentage complete text stopped at 99% and the time remaining stuck at -0.
Phone updated okay though but took 90minutes.