Because these flights happen in the real world, and they are NOT POSSIBLE if the Earth were flat.
The actual distance, using real-world great circle navigation is 6,863 miles.
http://www.gcmap.com/mapui?P=SYD-JNBA Boeing 747-400, flying at a normal cruising speed of .85 mach (about 580 mph), can complete such a flight in approximately 12 hours, depending on high level wind conditions. Qantas flies this route 6 days a week, as Qantas flight 63 and flight 64, depending on which direction you're going. 6,863 miles is within the published range of the 747-400 of 8,357 miles.
Now if the world was flat, we can calculate the distance between the cities using known coordinates. Both flat and globe Earth proponents agree on distances going straight north/south. Travel 1 degree of latitude due south, and you've moved ~ 59 miles. So Sydney can be found at 33.8688° S, 151.2093° E. Johannesburg can be found at 26.2041° S, 28.0473° E. What we know, and is not in dispute, whether the Earth is a globe or flat, is that Sydney is 123.8688° south of the north pole, which is 7,308 miles south of the north pole. Johannesburg is 116.2041° south of the north pole, which is 6,856 miles south of the north pole. Sydney's 151.2093° E minus Johannesburg's 28.0473° E equals 123.1620° of separation between the cities, as observed from the north pole.
Using trigonometry, or even easier, a triangle calculator like this one:
http://www.calculator.net/triangle-calculator.html, we can input the known values of the triangle formed between the three points of Sydney, Johannesburg, and the north pole. We know one angle, the one at the north pole, as well as the length of 2 of the three sides: NP to SYD and NP to JNB. Now in the interest of fairness, it is clear I misremembered the last time I did this calculation, because I thought it was about 14,500 miles, but our results are a distance from SYD to JNB are still 12,027 miles. IF the Earth is flat.
So for any aircraft to make the 12,027 flight from SYD to JNB on a flat Earth, it needs to average a smidgeon over 1,000 mph. A Boeing 747-400, which is the ONLY aircraft Qantas uses for this flight, is incapable of supersonic flight (the speed of sound at sea level is about 730 mph, and slower at airline cruising altitudes.
100% sure card ignores this post.