One big issue is that colloquially "the flu" is used to describe bad colds and other illnesses. Almost no one who has "the flu" has influenza. Thus people underestimate how bad influenza actually is
Actually I believe that in itself is a misunderstanding. Flu may be more likely than a cold statistically to hit someone badly, but it's quite possible to get the flu mildly, and it's quite possible to get really, really bad cold. The same way Covid being statistically more dangerous than the flu doesn't mean that every person getting infected by Covid has a worse time than every person getting the flu.