So, firstly, because you can't actually do that. If
literally everyone stays at home for three weeks, then there is nobody to staff power plants, water plants, etc, and those services fail. There are also no ambulances to help people who fall ill at home, no hospitals to take them to. No police to chase after people who decide to break lockdown and loot some deserted shops or prey on isolated families. No firemen to respond to accidents, wildfires, and arson. All of this kills immense numbers of people - the death
rate might be low, but you're locking down eight billion people here.
And it's not just these people. It's all the people providing the services they need to do their jobs. Police station cooks. Ambulance mechanics. Hospital laundry staff. Couriers delivering spares to power stations. And the people they depend on: catering suppliers, cleaning supplies wholesalers, warehouses. And so on, the transitive closure of everyone depended on by essential workers. Which, because of the ramified and interconnected nature of a modern economy, is a substantial fraction of everyone.
Secondly, because even if you did that, not everyone would do that. Some people would break the lockdown to see their dying relatives, suicidal friends, starving neighbours, star-crossed loves, bridge partners, etc.
Thirdly, because even if they didn't, you're locking down whole households, and you can get serial transmission between individuals in a household, so SARS-CoV-2 can last longer than three weeks, it can last about two weeks times the number of people in the household (assuming there are no reinfections).
Fourthly, because even if it didn't, somehow you blocked intra-household transmission, in a handful of cases, infections can last longer than three weeks in a single person.
Fifthly, because even if they never did, SARS-CoV-2 has several animal reservoirs in wild species which live in close proximity to humans.
The idea of "three weeks to stop COVID" was always an absolute fantasy, and honestly, very obviously an absolute fantasy. I am not an expert on viruses, epidemiology, logistics, public health, anything like that. But i read the news and i thought about it for a few minutes. What i don't understand is how so many other people didn't.