If you think about it, these space agencies would have to spend a lot of money to keep this "secret". Even if we didn't go to space, space agencies would still need to convince people that they did.
1. Making the rocket would cost millions and billions of dollars. You'd have to hire people who know physics of the how the rocket would move. You'd have to hire people who have studied and are well educated in rocket science. You'd have to hire engineers and mechanics to build the actual rocket. Then you need all the material to construct the rocket. You need to get the correct kind of gas/gasses to launch the rocket. Then this whole process would need some test runs before actually launching the rocket.
2. These rocket launches happen more often than they seem. They are at least 5 that will happen in this year alone with both manned and unmanned launches. These launches can be seen by anyone who wants to visit and watch.
https://www.nasa.gov/launchschedule/3. All satellites are fake, according to FEers. The satellites, that literally anyone from anywhere can see, FEers say that they are just “pseudolites or stratolites” put there to fool us. are just put up there to fool people. Satellites move due to the (round) earth’s orbit but if they are just fake, they need to be moved like a plane flying through the sky. These “fake satellites” are just being manually moved, but to do that even more money would have to get used. More money would be put in to build the satellite, hiring people to make it and hiring people to operate it. NASA would have to be constantly spending money to put out these “fake satellites” that supposedly fool people.
4. Hiring people for "special effects" when the missions happen would cost a lot too. There would be thousands of people needed to film, editing, add in the "special effects" and on stage assistants. Then there would be people who work on the costumes for the films. If they were filming underwater (as some flat-earthers say) you'd need another crew to film in those conditions. If they were filming in a zero-gravity plane (as some flat-earthers say) you would need another crew for that as well.
5. After all of that, you would need to pay those people to keep their mouths shut. You'd have to pay the physicist, engineers, and mechanics for the fake rockets. You'd have to pay the film crew, underwater crew and plane crew. You'd have to pay the editors. You'd have to pay the special effects supervisors and those work with them. You’d have to pay the satellite builders and operators.
That kinda money wouldn't just be a couple thousand bucks. That kinda money would have to be enough to financially support each and every one of these people's lives and their families. Then it would have to also cover for all of the luxuries the people would demand in order for them to keep silent.
6. Not only would this apply to NASA but also to other spaces agencies like CNSA, ESA, ISRO, JAXA, and Roscosmos, which in total have over 150,000 employees. All of these would also have to get paid a substantial amount of money as well. Not only them but the 70 different government space agencies, over 30 private spaceflight companies and over 20 private endeavors would all need to be paid.
7. Along with the NASA employees and other space agencies, some flat-earthers have said that they pay influential people, like Neil DeGrasse Tyson, Bill Nye, Richard Dawkins, Michio Kaku and many other influential scientists, to say that they believe the earth is round. That is not just a couple hundred but up to the thousands of highly educated people also believe an in a round earth. All those people would need to be paid as well.
NASA would end up making all of these expenses into faking this whole thing. After making the expense after expense, at the end of the day, the money that is left for the people who started "faking" wouldn't even be worth it. Those people who are "faking it all" wouldn't really benefit in the end.