Does it matter what I say? If I say I haven't, the FE response is "because it's not there". If I say I have seen a curved horizon, the FE response is "that's because you have been brainwashed by freemasons to think you see a curved horizon". So no, it doesn't matter to Dogmatic Flatearthists what answer we give. Also, I am not about to base my belief in the shape of the world on what my tiny perspective is of what I see in front of me, especially given the enormous size of the Earth. This is not proof at all of FE.
You're right it's not proof - at least not absolute proof - for that sort of proof we go to scripture, tradition, and science.
The sort of proof that it does lend itself to is that we cannot personally see any curve whatsoever, though some of course have said that they can see a curve from planes.
But from our little personal perspective, which, granted, is limited, we can still say that it is infallible that we cannot see a curve.