What makes you think that? AFAIK, no evidence has yet been presented of transmission to humans during the incubation period.
Symptoms aren't displayed till massive infection is present.
Infection is present (and transmissible) even though it hasn't reached massive symptom levels yet.
People BREATHE. The viri ARE in the breath/lung/mucosal moisture micro droplets! Maybe not in massive symptom amounts, but still PRESENT. It may seem like the virus has a starting pistol of symptoms to be transmissible, but factually looking at reality bears it to be much earlier.