Why is a 14 day period of self isolation or quarantine generally recommended? Do the chances of infecting others presumably fall after that?

The incubation period (time between when you are exposed and when you have symptoms) is 2-14 days.  If you have been in quarantine for 14 days and are not showing symptoms, you most likely are not infected.  (“Most likely not” because some people are asymptomatic.)

The thought is that once your immune system has developed the antibodies, your body will purge/consume the viruses.