Earth is round for the same reason other large planets are round: gravity pulls material toward the center until the body becomes a near-sphere. Earth is not a perfect ball - it bulges slightly at the equator - but it is very far from flat.
There are a lot of independent checks. We can see Earth’s curved shadow during lunar eclipses, watch ships disappear hull-first over the horizon, and measure the planet’s shape with satellites and geodesy.
You do not need a complicated theory to explain any of that. The simplest explanation is the right one: Earth is a rotating oblate spheroid, and we have been measuring that for a very long time.
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