Because the effect doesn't really become significant until middle-age, and while the slope rises fast from there, it was always understood to be something that

Because the effect doesn't really become significant until middle-age, and while the slope rises fast from there, it was always understood to be something that all but the oldest of immune systems could handle. The shutdown was initially to protect health services from becoming overwhelmed. Although it was later re-interpreted as you suggest, that was never a purpose grounded in either good science or evidence. Although it would have been hard to protect the virus from spreading to the middle aged, we could very easily have limited its spread amongst the oldest and most vulnerable. The size of the group is much smaller, and it would have been easier. "All" it needed was for people to focus their protection efforts. As it was, we tried to protect everyone. It was a waste of resources, and far worse than that, it failed to provide substantial protection to anyone.