You can't quarantine vulnerable people completely. If everyone gets it, every bag of groceries is going to be covered in COVID and an enormous number of quarantined people will die.
You can't tell who's going to die from it.
You can't have everyone getting sick at once. The death rate will go up tremendously if nobody can get treatment when they have treatable symptoms. This is what "flattening the curve" means.
We don't have enough protective equipment, medical supplies, hospital beds, healthcare workers, or even morgue space. The only tool left is staying inside, and so that's what we're doing now.
"Herd immunity" uses the word "herd" because we don't normally use it to describe actions taken toward human beings.
Sweden absolutely bungled their response and have 10x the number of deaths from this as compared to their neighbors who shut shit down.