Probably because many, if not most, players don't find it enjoyable to track diseases, let alone mundane diseases. I'm guessing people don't find it fun, and typically like to spend their time having fun.
Did you ever have a pc level 3 or higher die from a not thrid party or home brew disease?
how to you make disease threatening for PC NPC and civilizations?
how fast do disease spreads in a city like waterdeep per day?
or in a places phandalin?
