I think the phrse must have originated whe someoen was looking at a map.

Isaiah 11:12 And He will lift up a standard for the nations And assemble the banished ones of Israel, And will gather the dispersed of Judah From the four corners of the earth.

Revelation 7:1 After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth, so that no wind would blow on the earth or on the sea or on any tree.​​

You can blame the guys who wrote the Bible

Because there is a north wind, a south wind, an east wind and a west wind. The Bible clearly says: Rev: 7:1 After this I saw four angels standing at the four corners of the earth, holding back the four winds of the earth to prevent any wind from blowing on the land or on the sea or on any tree.
Although that somewhat conflicts with the circle of the earth spoken of elsewhere, and the circle had a firmament above it.

It's just another one of our idiotic sayings like "you can't find a needle in a haystack".

I guess because an oblique spheroid doesn't have corners and they are trying to project thoroughness? lol

Earth reflected in a drop of water, in front of a map of earth

Not if you look at it on a map. A map of the world has corners.

That's an example of local versus global perspective.

I believe the phrase is either older than the ancient greeks, or was created as a reference to maps, which will have corners.