The halachic validity of civil marriages

Hello Rabbi,
I am currently studying for the Rabbinical exam in the field of wedding ceremonies and kiddushin. The Shulchan Arba (Ahaza 26:1, B.A. and Ben-Ach) explains that someone who does not observe Torah and mitzvot does not fall under the rule, "No one makes his excuse for fornication." Therefore, if a man and a woman live together as a couple for years, and even if they married each other in a civil marriage, the woman is not considered the wife of a man.
This is the language of the Rema: A Gentile who marries a Gentile according to Gentile etiquette, and a converted woman who marries a converted woman according to their etiquette and later converts, there is no concern here about sanctification at all, and it is permissible to leave him without a get, even if he has been with her for several years, it is nothing more than adultery.
We know that even among Gentiles, a marital system exists, even from a halakhic perspective. After all, one of the 70 commandments of the Noahide is fornication, and a Gentile who comes to his fellow man's wife is liable to death.
My question is, why is the relationship considered 'fornication with a husband' when the marriage is not halakhically anchored, as defined by the Rema? Why wasn't it said that even if the halakhic level is not present here, the general human level exists, and the man and woman will be considered a couple at least on a gentile level? Why wasn't it said that as long as there is no clear separation (civil divorce), the woman is considered the wife of a man who will commit suicide?