Because any evidence we provide flat earthers is labeled as a fake picture or a fake video or part of a great conspiracy. Every photo taken from space: fake. Every video from the International Space Station: fake. Every single GPS, military and telecommunications satellite: fake. The stars rotating the opposite direction in the southern hemisphere: fake. Ships shown disappearing hull up over the horizon: fake. Airline flights taking just as long to travel similar longitudes in the northern hemisphere as the southern hemisphere: fake. Snipers taking into account the Coriolis effect of the earth for long range shots: fake.
Apparently the conspiracy is so widespread that it includes:
All space agencies across the world
All private space companies in the world
All military intelligence agencies
All telecommunications companies
Anyone who has taken photos/video in the southern hemisphere
Anyone who has spent a lot of time sailing around the world (to include members of the Navy and all shipping companies)
All airline companies
All airline pilots
Military snipers and professional Marksmen
99% of the scientific community
Hell, even basic math will tell you the earth can't be flat (as detailed in my post:
https://forum.tfes.org/index.php?topic=6120.0). Honestly, how much more evidence needs to be provided? What would it take to convince you?