It’s not just one thing. Near sunrise/sunset, sunlight passes through more atmosphere, which scatters more blue light out of the direct sun and makes the sunlight itself appear warmer/redder. That color-temperature shift is a big part of the effect.

At the same time, the light is usually softer and more diffuse, with longer shadows and often a lower lighting ratio than harsh midday sun. That can make scene colors appear richer and more pleasing.

For photography, there’s also a capture issue: golden-hour scenes often have lower dynamic range than bright noon scenes, so cameras can hold both sky and foreground color more easily in a single exposure. In harsh midday light, you may have to choose between exposing for a vivid sky or a properly exposed foreground, which makes the final image look less saturated overall.

So the perceived “extra saturation” is usually a combination of warmer light from atmospheric scattering, softer illumination, and a scene/camera contrast range that is easier to record well.