Explain why the earth is not perfectly spherical

But it is very close to a perfect sphere.

The longest diameter - the line through the center of the earth whose endpoints are on the equator - is only 0.3% (3/10 of one percent) longer than the shortest diameter that goes between the poles. And the variation in the surface from the highest point above sea level (the top of Mt. Everest) to the lowest point below sea level (in the Marianas Ocean Trench off the coast of Japan) is only about 0.15% of the mean diameter of the earth.

Yes, technically it is an imperfect oblate spheroid due to its rotation about its axis which causes it to bulge a bit. But it is very close to a perfect polished sphere.

Tropical climate is experienced in the nothern countries of south america