It was a panic reaction that gained too much inertia and became too useful. Most western response plans for dealing with a pandemic assumed it would be influenza, which has well-studied transmission properties, infection rates, and outcomes, and a certain level of base immunity within the population. Under those plans, the protocol would be to hospitalise severe cases, keep less severe cases quarantined, and attempt to achieve herd immunity, relying on the initial base immune response to speed things up, until a more permanent solution was found.
That's why the initial response was to put severe cases on ventilators. A flu can become severe enough to require a ventilator for a short while, because it clogs up your lungs and makes breathing difficult, but it doesn't damage them in the way that a severe coronavirus infection does.
The problem is, it didn't work. Coronavirus fucks your lungs hard if it gets a strong foothold there. It's effect on the immune system is different from influenza as well; it almost turns your immune system against you, though it isn't quite an autoimmune disease itself. The result is your lungs getting turned to mush and left vulnerable to subsequent pneumonic infections. Flu doesn't do that, except in cases so severe that you're dead no matter what happens.
And we don't have the same base immunity to corona that we have to flu. There are some relatives lurking around, but this one was novel enough to spread rapidly, unhindered, and largely unnoticed, for months.
But I said it was a panic reaction. By itself, this coronavirus isn't all that deadly. If you reduce the initial viral load, it basically doesn't do anything to you except reproduce enough to pass on to someone else. The panic was primed first by China's response to the virus, welding people into their homes, and then by the complete collapse of Milan's healthcare system, as the virus started showing up. Hospitals there were already under strain on the best of days, overcrowded, poorly funded, and with long waiting times for services. The influx of an unexpected number of very ill people, who all seemed to require ventilation, overwhelmed their resources and created a perfect storm of panic-inducing death porn. The sigh of a ward filled with people on ventilators created the impression that this was a highly contagious disease with a high death rate.
Everyone assumed it had just arrived that month, and was going to tear through the population like a wildfire. That modeller faggot in the UK, who had been demanding lockdowns every time someone sneezed for the last 20 years, got his model into the limelight and boosted the panic with wild predictions of millions of deaths worldwide, even if everyone was locked in their homes for months.
After that, it was pretty much inevitable that politicians worldwide would respond by locking everyone up. The virus wasn't reacting to their national response plans in the way it was supposed to, normal treatments just seemed to make things worse, and doomers started appearing with predictions of terrible fates for everyone that caught it. Locking everyone up was the only thing they could think of, and once one or two countries did it, everyone else followed along like panicked sheep. The ad-hoc strategy of "flattening the curve" to prevent overwhelming the health services, was used to justify the lockdown. That strategy itself was based on experience of Milan, which as I said, was already on the verge of collapse even before severe cases showed up. It only made sense if you assumed the majority of cases would require long-term hospitalisation and ventilation.
tl;dr the why is: politicians are human, saw a scary situation, panicked when their initial plans didn't work, and went full retard. Now it's become convenient to perpetuate the current state of affairs.
www.cdc.gov
