Quarantine is very difficult to implement effectively. Being quarantined interferes with people's ability to work and to care for family members even if they are not sick themselves. Knowledge of a quarantine tends to keep people from seeking medical care for themselves and family members, which allows diseases more time to spread through a community and tends to increase morbidity and mortality. Also, for many contagious diseases, once a case is identified, it's already been spread. The swine flu outbreak a couple years back offered a great example of this. Nervous elected officials called for closing borders to limit disease transmission, but the disease had already crossed borders and was spreading through populations without help from tourists and immigrants. There would have been no point.
Obviously, it would be great for public health if people stayed home when they were sick. Quarantines impose limits on more people than that, because you have to if you're going to keep the disease from being transmitted while it's pre-symptomatic, which is when a lot of diseases are contagious.