That's sarcasm but let me try to address the notion behind it, to best of my limited personal understanding; "prevent spread" has multiple factors to it.
My impression, in my locality, is that at beginning of pandemic people focused on notion of using masks to protect themselves (and many though not all governments indicated that's not efficient/recommended).
Then "as time passed and we learned more" focus moved to using masks to protect others (though many individuals, in my circle, still aren't clear on that).
My impression is that we have evolving, and still not necessarily 100% certain evidence/understanding, on how it spreads and what are the most effective measures. It's made more complex because
a) there's no silver bullet; most measures increase your chances to some percentage. This makes discussion between experts and public more difficult as public tends to think in binary terms.
b) While yes there are many public health measures we've known for 100 years (wash hands, have clean water, cook/boil/heat things to sterilize them, sneeze in elbow/Kleenex, wear mask, remove waste, etc etc), not all are equally effective against all vectors. What seems "Common sense" / "Logical" to a layperson like myself, may be more nuanced to an expert with experience.
I mean, for what it's worth, I'm 100% certain my dad, a year in, is still worse off for using a mask because of how he uses it. Many and especially older people around me reuse their masks for days and weeks , touch them constantly, put them under their nose for prolonged periods, don't squeeze/tighten them sufficiently, etc. Even if all that touching doesn't hurt, their belief that they're protected coupled with incorrect usage coupled with likely increase in risky behaviour is a net negative.
People can scream liberty and freedoms and personal responsibility, but I feel public health officials have to look at cold hard facts, of both disease but also people's actual behaviour (as opposed to some ideal non-existent form) and how it actually affects spread rather than how it logically intuitively should.