Because that would require global synchronised and incredibly intensive lockdown of the type that has never happened ever before in history. You'd need to eradi

Because that would require global synchronised and incredibly intensive lockdown of the type that has never happened ever before in history. You'd need to eradicate it globally and simultaneously including in war zones, third world countries etc. That's implausible, which is why the only thing that can work is to just let it spread and deal with the fallout. Sometimes called "herd immunity".