My current theory is that people are just scared. The numbers seem scary (no baseline as things like the flu are not nearly as thoroughly reported by the press) and every new infection and fatality is extensovely covered, whle the bigger picture isn't. Then this whole thing is just one big unknown, and humans are programmed to fear the unknown. Then you have confirmation bias, people tend to seek information confirming their opinions. Social media is designed to reinforce this bias, media is ware of that an needs clicks.This further reinforces this effect.
Also people want confirmation, so they actally seek out these filter bubbles. Humans in general also tend to be bad at risk management and evaluation. So, psychlogically I suspect it is just fear, combined with a desire for confirmation. Because, as crazy it is in my eyes, sharing your fears makes it easier for you to bear. Others not being affraid and giving you pushback on that is the last thing these people want or need.
I have still no idea how tackle that, spreading fear is still just plain wrong. Pushback and facts don't seem to work, so no idea what would.
But please compare/contrast the current state of play:
1. https://news.sky.com/story/coronavirus-they-call-it-the-apoc...
2. https://www.bbc.co.uk/news/av/world-us-canada-51955362/us-st...