The mixing of finances can be traced back to coverture where the wife lost legal personhood upon marriage. In that case the wife was not able to own property in her own name.
This doctrine developed into the various marital property laws we have today. Of course the wife no longer loses legal personhood, but the married couple itself still retains a kind of legal personhood, a couple can own property jointly, and when the marriage is dissolved the marital property needs to be redistributed back to the “shareholders” by a court.
At the same time, another change was happening: the cultural acceptance of divorce. While not so long ago, divorce was unthinkable for most, in modern times it’s completely normal.
So the law of marital property has developed based upon a model of lifelong marriage, whereas culture has embraced divorce and remarriage.
There is another culture that has a cultural acceptance of divorce and that’s the Arabs. But because they developed their marital property law afterwards, with the prospect of divorce and remarriage built in, it turned out in a completely different way. Instead of viewing the married couple as a single legal person (i.e. marriage as a corporation), they viewed the couple as individuals with a contractural arrangement, forbidden to interfere with each other’s property.
