Rather than simply accept what we are told, we observe the would around us for facts.
Unless it's Rowbotham or the word of another FEer.
I'm still waiting for what "facts" FE has other than the old look out your window bit.
And obviously pictures of the earth from space aren't all there is for RE. We can observe the curvature everywhere on earth with the old sinking ship effect, which is probably what moved the Greeks in the first place to believe it was round.
Beyond that, we can see the distance to the horizon increase with altitude, even without obstructions. The horizon itself exists, too. The distance doesn't gradually fade away, unless it happens to be foggy, of course.
Satellites orbit the earth, some in geostationary orbit. Sputnik was the first, and it was easily detected by its radio signal, even the United States. It was designed that way specifically, so people would know the Soviets weren't lying. People could actually measure the strength of the signal and find that the peaks were, in fact, an hour and a half apart. The same length as the time it took to orbit the earth. And the peak signal times matched when it would be passing closest.
Geostationary satellites, as I've mentioned elsewhere, can and have been photographed. You could even photograph them yourself if your location is right and take the angle measurements and see how high up they'd have to be, even on a flat earth.
The height of the sun matches with a round earth. We can calculate its height using a round world model and the answer is the same no matter what latitude we're at. If you use a flat earth model, simple trigonometry should be enough to find the height of the sun. Unfortunately for you guys, the answer changes depending on what latitude you measure from.
FM radio signals are blocked by the curvature of the earth. They're stopped right where the horizon would be for an observer looking out from the transmitter. How curious.
AM radio signals, on the other hand, travel much further because they bounce off the ionosphere. We can detect the ionosphere with radar, also, and it's curved. How odd. Why do AM signals go further than FM signals on a flat earth? If the atmosphere is all that's stopping them, shouldn't they go the same distance?
So yeah, you look at the facts and it doesn't look too great for FE. Of course, you're not truth seekers, you're denialists. Rather than go with what actually fits, you'll come back with something about bendy light, or some other phenomenon that you have no evidence for so you can keep clinging to the idea of a flat earth.