Why is the geomantic omen of U-shaped building not good?

There are many quadrangle-style private houses in the south of China. The form is that the surrounding houses are connected together, and a small courtyard is enclosed in the middle, which is called "quadrangle" private house. There are two basic forms of "patio courtyard" private house, one is composed of three rooms and one wall (U-shaped), and the gate is opened on the high wall in front; The other is surrounded by houses.

Reference pictures are taken from ancient buildings in China.