Why wouldn't we?
Well, because it has a massive and damaging impact on people’s lives, and even if you only cared about health, health is more than the absence of one specific virus?
“Sorry you’re going to have to spend your last three years in a care home isolated from all those who love you, Mum, but you know visits and hugs and seeing our faces is just far too dangerous. Maybe we can have one of your children come and wave at you through a window sometimes? We’re doing it for your own good. No you don’t get a say in your own good, Mum, don’t be so selfish!”
But even if you’re all right with that: what do you think happens at the end of those years? Because we could lock down for twenty years, massively wreck people’s lives, and then unlock… and coronavirus will still be there.