Because this is the first time that these sorts of stats have been used to curtail everyone's freedom so much.
We know this because we have seen how things have unfolded in places like Northern Italy and Iran - the hospital system is quickly overwhelmed, and then people start dying by the dozens in each hospital, because they simply can't get any kind of care anymore.
Another possible interpretation of this is that lockdowns don't really help much.
> We know this because we have seen how things have unfolded in places like Northern Italy and Iran
I thought those places did have lockdowns.
Famously, the Iranian health minister was on TV announcing that the country doesn't need to take special Covid19 measure while visibly sweating because of the fever caused by his Covid19 infection.
If you want positive proof that lockdowns work, the absolute best example is Vietnam: a tiny country of 100 million people which has experienced 1123 total infections so far, with 37 deaths.