Missing GA Routes

[VinPav]

With GA Aircraft I only get to see where they've departed from but not their destinations. This also goes for services like Royal Flying Doctor and Air Ambulance. For Commercial airline flights there is no issue. In other flight tracking software I can see the destination and departure points for these GA aircraft.
I'm located in Australia and looking at Australian flights.

[Oblivian]

Others may use/query flightplans

Here, if it isn't on the explicit list of known commercial routes they buy it doesn't know

It has a guess, based on first/last 'radar' receiver to 'see' it. Or uses the last route used with that callsign as the details. That can be way off if it goes past into an area with no coverage or known small airport.

So if you have a GA that uses it's reg as a callsign. It has no idea. Other than to guess based on the last flight it did. If it has the same set callsign, say FDR02 for flying doc 02, again. Same scenario. It has to be an explicit known route, or it will just use the last time it had that

[Kpin]

Scheduled traffic operate according to flightplans. They are accessible to FR24 in some counties, but not All. ga may or may to choose to file a flightplan. In either case it is genrally not broadcast (the US may be an exception).
Scheduled traffic will also broadcast their flight number as the call sign in the transponder data stream. FR24 have access to all (or most) scheduled flights with departure and destination airports and timetables.

When information is missing to FR24 they will make a guess. They might guess the departure airport to be the nearets to where the aircraft did first appear. And they may guess the destination airport based on where the aircraft flew last time. They are often mistaken - to the point where it makes absolutely no sense.