Why are the lymph vessels normally clipped during a lymphadenectomy?

Usually not: when doing a lymphadenectomy, the nodes are usually found along the major venous systems such as femoral vein, vena cava, axillary vein, jugular vein. Here it is essential to clip to venous structures and arterial branches feeding the nodes. One does not specifically look for microscopic lymphatic channels which for the most part travel along with the venous structures and are clipped as such.