Because when you look at the excess deaths charts [1], there's nothing at all surprising before mid-March. Then from mid-March onwards, deaths (from all causes) go through the roof.
So there simply wasn't any death toll before mid-March that suggests at all that the virus could have been spreading much earlier.
Likewise, we would have seen the hospitals becoming overwhelmed earlier.
Any unchecked spread in the fall or January would have caused those charts to have started spiking much earlier. There's no evidence for earlier community spread, and good evidence against it. You can't have a virus that can spread widely, and only later turn on its lethality.
So there simply wasn't any death toll before mid-March that suggests at all that the virus could have been spreading much earlier.
Likewise, we would have seen the hospitals becoming overwhelmed earlier.
Any unchecked spread in the fall or January would have caused those charts to have started spiking much earlier. There's no evidence for earlier community spread, and good evidence against it. You can't have a virus that can spread widely, and only later turn on its lethality.
1. https://www.nytimes.com/interactive/2020/04/28/us/coronaviru...