1. Airlines will try to stay close to land in case of an emergency landing, and will cross vast oceans where no other option exists. I think its called ETOPS. Basically means they have to stay close to land.
2.The northern hemisphere is a lot more crowded than the south, as such airlines will make a better profit by stopping in big hubs and picking up and dropping off people. So a flight goes from Cape town to Dubai with a plane full and drops off say half ( made money here ) they then pick up other people and then take them onto MEL or Syd and there again is more profit.

Flying for long durations above the ocean is dangerous, especially when it is not close to land. clouds can be made and in case of an emergency there is nowhere to land.

Actually, a direct flight between Cape Town and Australia is on the horizon according to this link: https://www.thesouthafrican.com/news/cape-town-to-australia-flight-time-direct-flight/

It's basically all about the money...
There's enough planes certified for long distance flights. E.g. 747s (or 787s) are used on the route between Sydney and Johannesburg (flight QF63) or A330s - between Perth and Johannesburg (flight SA280). Cape Town Air Access planned to launch direct flights to Australia last year, hoping for active tourist and business traffic. I believe that sooner or later we will see more such flights being operated between continents, despite all the problems in the airline industry this year.

The sea distance discourage the long hours of flight. This is due to atmospheric pressure which is no favorable to human breathing.

Richard E. Byrd is a person who flight over Atlantic and find a worm land. this is the world top hidden secret in the word
for more info
https://en.wikipedia.org/wiki/Richard_E._Byrd
is earth flat or not
some says it is end of the world
for more info
project dominic operation fishbowl america
Operation Fishbowl was a series of high-altitude nuclear tests

is earth flat or not
dave murphy has the all answer why air planes dont flight over thier
watch this video for more info
https://www.youtube.com/watch?v=LsqBaOdv_nY

sorry
i meant warm
Richard E. Byrd is a person who flight over Atlantic and find a warm land. this is the world top hidden secret in the word
for more info
https://en.wikipedia.org/wiki/Richard_E._Byrd
is earth flat or not
some says it is end of the world
for more info
project dominic operation fishbowl america
Operation Fishbowl was a series of high-altitude nuclear tests

is earth flat or not
dave murphy has the all answer why air planes dont flight over thier
watch this video for more info
https://www.youtube.com/watch?v=LsqBaOdv_nY

That route has the worse weather not only for ships but has the worse winter wind currents on the planet. Completely different than the Noth polar currents in the water as well as the air currents. The history of ships in that region shows in British Naval records from the first voyage Captain Phillips of the Navy took immigrant penal Colony prisoners in the early 19th century to Australia on the crossing in winter from South Africa to Eastern Australia. He lost one ship and had to scuttle a second. He just barely made it. Now, the airlines still have to contend with the West to East winds on Western flights. Very expensive fuel bills. On top of that the emergency landing is not available. That is the other huge factor. That are the facts of the matter at any altitude or season.

all about the money

My apologies, I should have said Eastern flights, all against the prevailing winds. That is a rough flight, almost as bad as flying from Rio de Janeiro to Lisbon in a four engine prop job in the early '50's.

It's called curved routes.
There are actually a number of reasons why airlines choose to fly this route:
1) Airlines try and stay as close to land as possible incase of emergency landings.
2) Incase oxygen levels run out for whatever reason (airlines have only 20 min of oxgyen) they need to descend to a lower altitude. This is called a drop down procedure. You cannot perform this procedure over the ocean since the ocean weather is unpredictable and storms in the middle of the ocean are quite dangerous. Hence they avoid flying over the ocean since oceanic storms are terrifyingly dangerous.
3) Contrary to what you can visually see on a flat map, Curved routes are actually quicker than straight ones. Remember, the earth is actually round, not flat as per your flat map image. They take into account, the curvature of the earth as well as its rotation when mapping out flight routes.
4) They will only stop at tourist hotspot locations. This is also a way of bringing tourism into other countries. Some airlines usually have contracts made up between them and a certain country. The basic principle of the contract is "I bring you tourists aka money spenders and you allow me to land and refuel."

I hope this has answered your question.

Hurm..why..