There are a few reasons for this, but the obvious one is that all the factory, worker, machinery, etc time that has been forcibly idled is gone, destroyed, everything it would've produced does not exist and no clever financial tricks can fix this. There's of course a bunch of more indirect effectis involving mass unemployment, the collapse of many businesses, the failure of demand for things, etc which are also going to be devastating. The US is likely to come out of this better than many countries - it looks like the deaths from this economic destruction are going to be co ncentrated in developing countries - but that's not much reassurance.