Here's why:

* The Sun's Path: The sun's path across the sky varies throughout the year, but it generally arcs from east to west. In the northern hemisphere, the sun reaches its highest point in the sky around noon, and this point is generally south of directly overhead.

* Maximizing Sunlight: South-facing solar collectors capture the most direct sunlight throughout the day. This is because the sun is directly south during the majority of daylight hours, leading to the most efficient solar energy collection.

* Seasonal Variation: While the sun's position varies throughout the year, it's important to consider the average angle of the sun. The average angle of the sun is tilted towards the south for most of the year in the northern hemisphere.

Why not face them east or west?

* Less Direct Sunlight: East and west-facing collectors would receive more sunlight in the morning or evening, respectively, but they would be less efficient because the sun is at a lower angle in the sky, resulting in more diffuse sunlight.

* Shadowing: East and west-facing collectors might be shaded by nearby structures or trees, further reducing their efficiency.

Note: The exact ideal angle for solar collectors can vary depending on the latitude, the time of year, and the specific design of the collector. However, the general principle of south-facing for optimal sunlight capture remains true.