لِأَنَّهُ الْأَصْلُ
Because separation is the basic state.
A newly uttered clause can stand on its own. Coordination adds an overt connecting element, so separation can be described as the basic state from which joining departs.
This historical or analytical priority does not mean separation is always rhetorically preferable. In a context that requires shared predication, omitting a connector can undermine the intended relation. The distinction is between an analytical starting point and the correct rhetorical choice in a specific utterance.