Could someone who knows what they are talking about answer this question?
It probably comes down to what causes the difference in reaction to the infection. It could be that there are differences in people's immune systems that cause them to have a much more severe reaction. In this case the quarantining wouldn't have an effect.
If people who are having a much more severe reaction are getting a strain of the virus that has mutated to cause these more intense symptoms then you would see the issue you are talking about.
My uneducated guess is that it is the former, but I'm really not sure.
From the evolutionary point of view, a virus has no interest in being deadly: if it kills its host, it can't reproduce itself.
When social distancing (quarantine and lockdown) is enacted, it is harder for a deadly strain of the virus to reproduce and hence survive, as we react to it by putting into place stricter countermeasures. Which strains will thrive? The ones that able to "sneak around" unnoticed, by causing only mild symtomps.
By the way, you misspelled "quarantine".