Why many live flights are missing at the aiport

It’s fun to follow a live flight’s flight plan and also part of an achievement . One thing I don’t understand is why is many live traffic missing on airports. For e.g I was looking at a flight that departs in 30 mins and it’s not shown in the live traffic. Many other flights are visible and also many just about to depart are not visible. I even used flight tracker to see the live flight status and it says it’s at gate and not departed yet in msfs the flight is not there. I’ve also set the Live ground traffic density to 100 (not if that matters). Or is it just msfs just shows random pick of live 10-20 flights on the airport at any given time ?

Moved to ATC, Traffic & NAVAIDs