While masks are far from 100% efficiency, they are still much much much better than doing nothing, while self-isolation for minor illnesses like a common cold is not in any way realistic.
Also, I'm explicitly not saying that we should always wear masks - the dangers of un-treatable, un-vaccinatable Covid (that 100% justified universal mask wearing) have passed. But, when you're sick and for various reasons have to be in closed spaces with others, wear a mask.
And before you say "I don't care if I pass my flu around.": Stay home /for your own sake/. What the fuck do you think is going to happen if your mild illness takes a turn for the worst right when you're overseas? I hope you have good traveller's insurance, or otherwise sufficient savings to afford medical and emergency care out of pocket.
Also, if you have mild respiratory symptoms (say, you're sniffling and your throat is a little sore), do you immediately go and run various tests to see what specific pathogen it is, or do you take some symptomatic treatment and go about your life? Why not add "and wear a mask outside" to that?
The irony is that the top 0.1% and the bottom 80% both have shit manners, and only when you make the top 20% act well and put on their shoes do you have something resembling civilization.
And plenty is lost about a society that focuses more about preventing illness than just staying generally healthy. Smiling is good for you, engaging with your fellow humans, with full facial expressions available for communication and connection, are absolutely essential to a happy and fulfilling life.