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

Some people see no reason to have a ceremony (or pay the government) for this.
They live together, are monogamous, and fight about money.
Sounds like a marriage to me.

Maybe because they are as good as married? They think of themselves in a relationship akin to marriage in terms of commitment but without the piece of paper?

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.

Because they feel like they are married in one way or another

They are trying to get to the next step up.

Many of us who grew up in the 70's remember a thing called "common law marriage". It's when two people live together for a long time it is then referred to as my husband/wife.

If it's been a number of years, it could be common-law.