Well, in simple terms, there are two options. Option 1, do nothing, leads to somewhere between 0.5% and 3.5% of people dying, plus other people dying as medical services become overwhelmed. Option 2, lockdown, hopefully reduces the direct death toll, stops health services becoming overwhelmed, and has a lot of side benefits too (reduced traffic accidents, reduced gun violence, etc.).
There’s no denying that there are costs associated with any option. Certainly the economy will be harmed either way, but much more with lockdowns. But at the same time, the damage done to human life by the lockdowns is vastly outweighed by the lives saved. If you look at the excess deaths associated with the Great Depression, they pale in significance compared to the deaths we’re already seeing due to this pandemic.
The costs of locking down are obvious to us because we live with them. The benefits are harder to conceive because they’re distant and mostly to do with potential things not happening. But there’s a reason why every world leader who isn’t a fruitcake has declared some sort of lockdown, regardless of their own personal political philosophies.