There are five common explanations for this, but in the end without toppling the Planar Conspiracy there is no real way to know. Different people believe different explanations, or even ones not mentioned here.
To Control Information: In a similar way to 1984, control of information is control of everything. The flat earth is even mentioned in the book itself.
The problem is who is THEY?
Because who ever "they" is will need friendly co-operation from every other authority in the world, including the likes of both Iran and Israel.
Unless you live under a rock, you will realize that few countries agree on anything.
They are wrong: simply said, we are wrong about the shape of the earth, and there is no conspiracy. This describes why the earth might look round from far away: https://theflatearthsociety.org/home/index.php/blog/einsteins-relativity-proves-earth-flat
There is so much evidence that the world is round, if it is anything except round the only other option is a matrix like simulation, where the world may be flat in another version of reality, but not ours.
The odds of a round world against a flat world look dim for the latter.
To Maintain Legitimacy: During the Cold War we faked the moon landing. Shortly after they realized the reason they could not reach the moon was due to the flatness of the Earth. They were stuck in a lie, and had to continue it or lose legitimacy of our governments. Even today we would still hold onto this lie due to role Science plays in our ruling government.
And why would the USA's sworn enemy at the time agree with something like that?
USSR was ready to nuke the USA, but happy to play along with this?
And then why would all the countries decide to play along? The world is bigger than the USA.
To hide the truth of the Bible.
only 1 third of the world is Christian. India and China have nothing to lose here.
To Gain Power and Money: By siphoning off the space budgets and denying the world the resources of the Antarctic they gain a considerable amount of power and wealth.
The USA space budget more than 4 times is smaller than Nestles annual revenue.
You can literally make more money selling chocolates and bottles water than taking all of NASA's money.