The problem is: we largely don’t know who’s “sick”.
People can contract & spread the disease long before symptoms emerge (if at all). Quarantining only the “sick” would require frequent & pervasive testing of pretty much everyone, identifying & isolating asymptomatic carriers; that’s great if you can, but practically we can’t (not enough sufficiently reliable tests available early enough in this mess). The risk was it would very rapidly spread thru society unnoticed, THEN see 1-3% casualties in its wake.
If we did only quarantine the sick, you’re relying on people getting symptomatic enough to get tested, which is long after they’ve spread it to many people close to them.
The only problem is that many epidemiologists are pointing out that the only way for a population to gain what’s called “herd immunity” is for the population to be exposed to small amounts of the virus, amounts below the threshold of exposure to trip over into full blown virulence to cause illness, but just enough to create antibodies.
By avoiding such minor exposure to the virus that never happens. What DOES happen is that when those who avoided exposure come out from their quarantine, a SECOND WAVE of exposure occurs and THEY get ill. it is better to have everyone be exposed and build that herd immunity.