earnest answer: because even if it had a 0% fatality rate, the sheer number of people who would be out sick for a week at the same time would decimate the economy. The bailout package is probably not going to be enough.
This is different to something like 2008 where people no longer trusted financial and other institutions.
This is more like pre-1970's recessions where the central banks squeeze the money supply for a couple of months. Or WWII mobilization. In each of those cases the economy came roaring right back due to pent up demand.
We will get the worst of both worlds if our response to this is to either a) let it 'run it's course' or b) try to make the finance sector whole. The US is looking at both those options and the result will be catastrophic of they come to pass.
the main problem right now is that there is a lot of fear and uncertainty in everybodys mind partly because of Italy & spain. and as cases mount that fear will only harden so regardless of whether there is a lock-down or not people wont go out an shop until a sizable portion of population is infected and recovered OR we determine with american data that CFR is indeed quite low (doesn't look like from hospital testing data & they wont do widespread tests).
but regardless I do agree bailout package is not gonna be enough.