To those people, odds are you don't get the flu because enough people around you did get vaccinated. And also, you're just getting lucky. You may have a better immune system than average, but you are not immune.

In 2018, the CDC says >490k were hospitalized and >34k died from the flu. https://www.cdc.gov/flu/about/burden/2018-2019.html

It's a serious matter, yet people just brushed it off and came in to work/school sick as a dog. Heck, I remember having to do this with colds in elementary school... It was either: I show up horribly sick, or my grades must suffer. (No, there was no way out of it. Simply punish those trying to protect their community from themselves. Zero tolerance for real life.)