Why do some people who are NOT legally married, refer to their significant other as husband/wife?

Not sure. In some places if they live as husband and wife (often called common law) they are still under the same legal obligations as a licensed marriage if one of them files for divorce. It has to be proven they were husband and wife if the other contests it, but people have done it and won.

Something for people to consider before stating somebody is their husband or wife. Don't take it lightly.