Control of space means control of the world. From space, the masters of infinity would have the power to control the earth's weather, to cause drought and flood, to change the tides and raise the levels of the sea, to divert the gulf stream and change temperate climates to frigid. There is something more important than the ultimate weapon. And that's the ultimate position. The position of total control over the Earth that lies somewhere in outer space.
—Lyndon B. Johnson, Statement on Status of Nation's Defense and Race for Space, January 7, 1958
That's basically it. The United States wants military dominance over space. NASA was originally created for this purpose, and interestingly enough NASA's original organization name was: US Army's Ballistic Missile Arsenal. This was later changed. NASA works on missiles and weapons to further American military dominance. Originally they wanted to put these weapons in space, but found out this was impossible because it is impossible to escape UA (Universal Acceleration). They maintain this lie to intimidate other nations into believing they are the supreme super-power on Earth... this also applies to any other space agency claiming to have gone to space. The reason they are telling everyone that the Earth is round is because they believe it to be round. They have never actually been to space so how would they know otherwise? It was incredibly easy for them to adopt the Round Earth theory because most of humanity already believes this due to faulty Ancient Greek teachings.
Whether or not they know the true shape of the Earth is a matter of much debate, but if they do they're certainly not going to tell everyone about it now that they're so deep in the lie.