There's a consistency to gravity pulling everything on Earth to a central point. Between the three main elements making up our planetary ecosystem, air, water, and soils/rocks, they all exhibit the same variations in density, equally, all over the planet.
Wherever you go, air pressure is always highest at sea level, and it progressively gets lower as you go higher.
Wherever you go, water pressure in seas, oceans and lakes is lowest at the surface, and gets higher the deeper you go.
Wherever you go, there are generally soils, sands and loose aggregrates at the surface, with denser soils and rocks as you dig down and go lower.
Why would this be universally so, if the world were flat? What possible explanation could there be for a consistent downward force in (say) Australia, and the UK, if they were both on a flat plane? Surely you would need a different focal point for the force in each location, otherwise the force would not act vertically.
EDIT
Wherever you go, acceleration due to gravity is found to be consistent with everywhere else on the planet. Which surely is more consistent with all gravity drawing everything to one point, as opposed to folks in Australia being drawn to one, and those in the UK to another.