Two reasons:
[ul]
[li]The mortality rate for COVID-19 is at least an order of magnitude greater than that of influenza.[/li][li]The number of cases of COVID-19 are increasing exponentially.[/li][/ul]
Note that “exponentially” is not a synonym for “a lot.” It’s a mathematical term.
When a danger is growing exponentially, everything looks fine until it doesn’t
The problem isn’t the current number of cases. It’s how many cases we will have in a few weeks or months.
More detail here:
Coronavirus: Why You Must Act Now