I was digging thru the forums and the FAQ in the manual, and I noted this:
Q: I sometimes see lots of aircraft parking at an airport but none taxiing, taking off or landing. What’s the
reason?
A: When starting a flight at an airport or when rebuilding the scenery database (e.g. after changing the day or time),
P3D/FSX builds a start-up situation of AI traffic at this airport, with all the aircraft parking and the approaching
aircraft some miles out. If you select the relevant ground frequency you will initially hear aircraft asking for
clearances (depending on the size of the airport and current activity) and a few minutes later you’ll see the first
aircraft coming in. When flying to an airport, however, the situation gets constantly rebuilt so you should see and
hear full traffic in the air and on the ground.