It’s strange how that happened. In English “good morning” is simply a greeting, same with “good day” (or g’day in Australia) and “good afternoon”. In context, they can also be a way of saying “goodbye”. But, if you’re working the night shift and you show up at work, you can’t greet your co-worker with “good night”.
The funny thing is that The Americans was actually based on a similar case from 2010. In that case the kids were 16 and 20 when they found out their parents were spies.