That's because many people are disgusting and don't give a fuck about proper hygiene. Just among my acquaintances I need both hands an a foot to count the number that don't even vaguely cover their mouth / nose -- let alone _properly_ cover -- while coughing or sneezing.
No amount of gentle reminder -> passive aggressive suggestion -> explicit suggestion had lasting effects on their behavior. All I could do is be vigilant and recluse myself from their company whenever one's clearly showing symptoms.