There is not in fact zero evidence thta any ancient culture thought that the world was flat, the early Greeks pictured it for instance as a disk surrounded by an encircling ocean.
The Egyptians depicted both the sun and moon as spheres in relief carvings, but did not voice any idea of the shape, nor size of the earth. We can INFER they imagined it as flat as they believed the sun had to travel thru the underworld to get to the other side of the 'earth' - but in fact they never actually describe the world in any way at all- they just took it for how it appeared.
People sometimes cite old maps that show a disk shaped earth... but, in fact, a circle is how ancient peoples depicted a sphere in flat drawings, so that is not evidence they actually envisioned the earth as a flat disk.
And people in the 1700s ridiculed people of the 13 and 1400s for voicing fears of sailing "Off the edge of the earth"- which they smugly interpreted as meaning they thought the world was flat- when in fact it was a nautical term referring to sailing out of sight of land when the only way you could truly know your position at sea was using portolan charts to fix bearings on headlands and landmarks.
Hegemonic empires resulted in people traveling widely and noticing certain odd things, like the sun being at different heights in the sky, or stars becoming visible or invisible beyond the horizon... and that and only that appears to have been the point at which human beings began to even consider that the world had a "shape" at all.
Prior to such experience of significant travel... there appears to be no evidence that people "pictured" the world as having any given shape or dimensions.
Once they started to give it any thought at all- it became clear that only ONE shape explained observations.
The Greeks figured out the sphere in the 6th century bc.
Flat earth beliefs are relatively modern imbecility, based upon stories about how people used to believe such things. When in fact they either didn't believe it was flat. - or they did not think of the world as having any shape at all.