Because 1080 people died from this thing in the last 24 hrs, and the infection and death rates are increasing logarithmically, and we have no particular reason to think that it'll stop before its infected 50% of the world's population (low-ball on herd immunity numbers). Which if trends continue will be 427,471,737 deaths. So there's only one number in the equation we can impact. R0 is the obvious one, if we can slow the spread enough, eventually it'll die out. Look at Singapore and China if you think that's not possible - it's not easy, but can be done.
If you think that the China model of outbreak stopping is maybe a bit extreme (hint, if you think NY's measures are bad...), the other option is to slow R0 less, but still enough to keep us from running out of medical stretch capacity and lower that 11% current fatality number, which is largely impacted by Italy's epic fail at managing their R0 numbers, much like you're advocating we adopt, so that the number of fatalities decrease.
All numbers taken from https://www.worldometers.info/ and I freely admit that I don't know everything, and can't see the future any better then anyone else, but those look like the best data I can find.
Now MAYBE we get lucky. MAYBE we find a good treatment in time. MAYBE it's seasonal and gives half the world (the one we're both probably in given we're talking on Slashdot) a break for 6 months. Maybe it mutates and becomes less infectious/less lethal.
I HOPE we get lucky. I PLAN that we won't. The thing about logarithmic/exponential curves is that you have two options - Overreact or Under-react. The thing about pathogens is underacting costs lives, you can ask Italy about that, there's no end of online sources about what goes wrong when you under-react to this bug.
So PLEASE over-react. I don't know you, but you're a human, and I want you to not spend weeks in a hospital bed, or see a loved one there. In the 1918 flu, everyone who survived knew someone who died. I don't wish that on anyone.
Min