Crikey, again, I myself have said similarly before! Lets see...
- The Spanish flu pandemic of 191820 infected 500 million, killed 50 million worldwide (at a time when the worlds population was much smaller, and commercial passenger air transportation did not yet existaeroplanes were still quite new technology)
- The coronavirus outbreak of 200203
- Other reasons I have no time to list here
Maybe there is your why. Just my own little pet theory. 