My math might be wrong, but if we were accelerating at 9.8m/s/s for at least 4000 years (roughly as long as we have continuously recorded history and the minimum time “gravity” has been observed) then we ought to currently be traveling through space at over 1,000,000,000,000m/s.
Now I’m no physicist, but I reckon that might end up violating causality.
The problem with trying to "explain" this is that fundamentally, flat-earthers, to the extent that they could be said to have a coherent world view at all, are usually a kind of occasionalist[1]. They don't _believe_ in natural laws or cause and effect. For the most part, they believe that god is in complete control of all events, and things go down because god wants them to go down. There's no required explanation for _anything_. The sun moves across the sky because god wants it to, and he could stop it or make it go backwards if he wanted it to, etc.
Indeed, that a flat earth is incompatible with physics is part of the appeal of believing in it to begin with. They _want_ to overthrow Newton, because a clockwork universe is incompatible with their belief system.
It's also sort of immune to any kind of argumentation. The result of any experiment is simply that god wants it that way, that they're predictable and testable doesn't _prove_ anything, because you can do an experiment a million times, and god could still cause it to fail any time he wants to. God just doesn't want to argue with Netwon right now, for his own reasons, you see.
I guess it must be a pushing force from below.
So, who's doing the pushing? I'm thinking a big turtle.