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Perhaps they believe their infrastructure is ready to handle the victims now. Here in the US graph that Dr Fauci keeps showing appears to make the assumption that we can’t limit the number of people who are going to get sick, we can only delay when they get sick so our facilities have time to ramp up capacity.
Edit:
Scott’s models below illustrate another effect of delaying the spread. At some point recovered individuals are able to work freely to get the economy moving again. Social distancing and delaying onset seems to make good sense.