Because they still contribute to chain of transmission to the most vulnerable members of society, likely in a disproportional way since younger members of society tend to socialize more. Fighting a virus is a collective action. In order to stop transmission to the vulnerable members, you need to cut edges along all paths through the graph. Furthermore, additional spread, even among healthy people with no side effects, increases the probability of mutations that lead to more fit variants capable of causing even more sickness and death.