Could be in part because of the separation of church and state, and that monogamous pairings under organized civil legal systems has existed far longer than you

Could be in part because of the separation of church and state, and that monogamous pairings under organized civil legal systems has existed far longer than your God and already had routines in place for dissolution of said pairings, especially in cases where one partner is harmful to the other, despite fantastic and ritualistic promises and vows.