The Pauline and Petrine privileges are not exceptions. They only apply to marriages where one or both of the spouses are not baptised, what are termed non-sacramental, natural marriages. If an unbaptised spouse decides not to live with his/her spouse because he/she is or becomes a Christian then the Christian spouse is permitted to contract a sacramental marriage with and only with another Christian. If the unbaptised spouse is content to live with his/her Christian spouse then the marriage cannot be dissolved. Presumably the spouses in a natural marriage are not joined together by God.
If there are no scriptural reasons for the indissolubility of marriage why would Our Lord choose to go against the status quo.
The binding and loosing power is Christ's guarantee that the Holy Ghost will ensure that God's Law and the Church's teaching will never diverge.