Why does Earth have uneven heating from the Sun?

There are two reasons: the angle of insolation and the albedo of the surface.*

The Earth's surface is curved, so that while one area receives the sunlight perpendicularly to the surface, other areas (notably higher latitudes) receive the sunlight at an angle. Not only does this increase refraction, but also scattering, because the sunlight travels through a greater volume of air on the way to the surface.

The albedo is the relative reflectivity of an area of the Earth's surface, or of cloud layers covering the surface. The higher the reflectivity, or the greater the cloud cover, the less solar warming of the surface. However, clouds at night can also trap infrared radiation re-rediated by the Earth, in some cases compensating for the reduced solar heating. (In the worst case scenario, daytime clouds would block the sunlight, then dissipate at night.)

Ice and snow are more reflective than other surfaces such as rocks, trees, and buildings, so winter sunlight provides even less heating when temperatures are very low and there is more snow on the ground.

* Water surfaces absorb more heat than land surfaces. But this is less a case of uneven heating than uneven absorption.