Uh, yes, you do.
1. To protect your citizens from a mutating your virus to adjusting into your age group.
2. To prevent the medical system from collapsing.
Theres no point in keeping an economy open if the medical system has collapsed and the workforce is ill at home.
Its not even revisionist history at this point, its that you are proclaiming to open everything up without knowing what happens. Even Florida put restrictions up outside of essential services.
And I am not suggesting the way how it was handled was correct. I am suggesting simply letting everything open and play itself is an easy strategy to take if the consequences arent fully known (no, Florida and Sweden are not examples).