It’s because the incubation period is longer than 21 days
21 days is the 99th percentile, not the limit, which is more like 25 days.
and... they are infectious BEFORE they show symptoms.
You need a link for that.
At the time he left Liberia, Duncan was infected but was not showing symptoms.
If he was infectious at that time, we should be seeing some new cases among his seatmates on the flights from Monrovia to Brussels, Brussels to Dulles, and Dulles to Dallas.
He was exhibiting every symptom to the point of apparent projectile vomiting by the time he left that apartment for hospital the second time. No new cases, not yet or not reported at least.