I am not suggesting that there are long-term asymptomatic carriers, rather that many, maybe a majority, have mild or asymptomatic infection. If an infection causes symptoms, the person self-isolates because he or she is too sick to move around and interact with others. Further, if an infection causes symptoms in a majority of cases, the outbreak is easier to contain, because sick contacts are easier to locate and quarantine. Once the cat is out of the bag, asymptomatic infection and spread makes containment impossible. But asymptomatic infection also leads to herd immunity eventually.
I am not suggesting that there are long-term asymptomatic carriers, rather that many, maybe a majority, have mild or asymptomatic infection. If an infection causes symptoms, the person self-isolates because he or she is too sick to move around and interact with others. Further, if an infection causes symptoms in a majority of cases, the outbreak is easier to contain, because sick contacts are easier to locate and quarantine. Once the cat is out of the bag, asymptomatic infection and spread makes containment impossible. But asymptomatic infection also leads to herd immunity eventually.